May I suggest not washing your hair with eggs when you've got a hangover? Putting your hands into a jug of the stuff, stomach still gently reeling from a late night out, is like a cruel bushtucker trial. And I'm not even sure it will clean my hair.

This is what happens when Mark Constantine, the head of Lush cosmetics, is in your kitchen. A polite inquiry as to how hard it would be to make beauty products at home has resulted in a masterclass, in what can only be described as gunk, from Constantine and his colleague Helen Ambrosen, who has been working with him from the very earliest days when he was starting up the Body Shop with Anita Roddick.

The two of them zoom into my kitchen, giggly with excitement at the chance to plough through my cupboards and see what they can come up with. Within minutes there are heaps of porridge oats, dessicated coconut, dried beans, bananas, mushrooms, ground almonds, a pot of honey and a forlorn little pile of flower petals, ripped cruelly from the bouquet they brought with them.

"Right," says Constantine. "Now, the first thing you have to do when making skincare products is not get into all those little boxes that the cosmetic industry wants you in – greasy, combination skin, all that. You have to think about exactly what you want to achieve. What do you want?"

"Erm . . . something to clean my skin. And hair." I reply

He stares at my hair. "Oily at the roots, dry at the ends, a lot of static, I bet, flyaway . . ."

While Constantine lists the various items in my kitchen that can be coerced into working on my terrible hair, Ambrosen prepares a rice roulade for my skin. I have said that I only want to use items that you would naturally have to hand, but she asks if she can include three reasonably cheap ingredients which, in one form or another, are the basis of many of the products you buy in the shops (you can buy all of them at your chemist). She has brought glycerine (this is a humectant, which means it holds the moisture in the skin – it is impossible to moisturise your skin with water only), sweet almond oil (for recipes that need a vegetable oil, this is the most useful) and kaolin clay (clays are vital for tightening and helpful for cleansing). Using them in different proportions varies their effect.

There is one ingredient she has not brought, however, because "the great thing about making your own products is that you don't have to use preservatives. All cosmetic companies must by law put preservatives into their products, but they're horrible things."

Constantine agrees: "Basically, they are there to kill any organisms that may get into your product, and for no other reason. What you have to understand about the modern cosmetic industry is that what the product will actually do is probably the last thing they're interested in. When they're putting a skin or a hair product together, they're calculating how long it will last, how it will travel, the cheapest way to make it smell nice. It's only after they've sorted all this out that they might have a little think about whether it will actually work."

Ambrosen rubs the roulade on the back of her hand. "Try this. Is it scratchy enough? How scratchy do you want it?" A little scratchier, I say, and we look around for something else to add. A large spoonful of dessicated coconut gets chucked in, and then some of the oil of evening primrose capsules I have lying about.

"This is the best thing about my job, just messing around with things, coming up with something great at the end, or just having to ditch the whole day's work," says Constantine. The roulade is nearly ready and smells unexpectedly delicious. We spend a couple more enjoyable hours fiddling about (to my great joy, the burned bits of a banana cake I have made are included in the face mask – charcoal is good for your skin) and then they zoom off again.

Using these recipes and ingredients, some of which work, some of which don't, is much messier and feels somehow more alive. The eggs, for example, leave my hair soft but with an unpleasant powdery texture that comes away on my hands when I touch it. I admit that I neglected to rinse afterwards with lemon juice, so this might be my fault. But for the next few washes – with normal shampoo – it comes up absolutely gorgeous, healthy and shining in the way it only does after I've been to the hairdresser.

My conclusion? Beauty products are like technology; sometimes the latest must-have is a sideways or even a backwards step, or just a bit of exploitative marketing. But sometimes they represent genuine progress. And so it is with beauty products. The cleansers you might buy in a high street chemist are pallid, synthetically perfumed, expensively packaged, and not a patch on the cleanser I made at home with some dessicated coconut in quarter of an hour, and which I am using still. Oats in my bath: yes, please. However, when it comes to washing your hair in eggs, a little progress is perhaps not such a bad thing.

Skin cleanser

Into a big bowl Ambrosen tips 2 big spoonfuls kaolin clay, 1 tbsp ground rice (wonderful for exfoliating the skin), 1 tbsp ground almonds, ½ tbsp wheat flour, ½ tbsp of almond oil and 5 capsules of evening primrose oil. She lets me test it – and we add a bit more ground rice and then some dessicated coconut. She looks wistfully at my dried beans and says that if I had a coffee grinder they'd also be perfect exfoliants.

Then she rolls it out to about an inch thick, and then rolls that up; the idea is that you cut off slices when you need to exfoliate (no more than once every two days). "Completely Coconuts", as we name it, will keep in the fridge for as long as you want it to. You could vary it by using an antiseptic herb such as thyme if your skin is a bit spotty, or fresh, finely chopped fennel, which is very stimulating. To make a good body scrub you might combine kaolin, glycerine, fresh pineapple (this contains the enzyme bromelain which is helpful, says Ambrosen, for getting rid of dead skin cells), ground rice and some sweet almond oil.

"This is how we start all our recipes," says Constantine. "Wandering along the aisles of Sainsbury's. Anita [Roddick] used to do all that travelling-the-world stuff, but the only useful thing she really ever brought back were some brazil nuts."

Face packs

Both Constantine and Ambrosen light up with joy when they see my porridge oats. Constantine says fervently that "oats are wonderful", which I thought was slightly over the top. However, I am now in complete agreement. They are wonderful.

Ambrosen makes an infusion of lily and honeysuckle petals by pouring boiling water on them and then leaving it for half an hour. Then we mix 4 tbsps of that in with 1 tbsp oats, 1 tbsp kaolin, 1 tbsp glycerine and all the burned bits of my poor banana cake (charcoal is apparently wonderful for your skin, although it is possible, I suppose, that you may not have a burned cake to hand). I try it out the same day as this is one that won't keep; it's slightly odd to use, as bits of porridge oat keep slipping down my face, but the next day my skin is soft as you like, and a spot that was threatening to come up on my chin gives up and goes away. By increasing the amount of clay you put in, you'll increase the tightening effect; conversely, if moisturising and softening is what you're after, use more almond oil and oats.

Constantine also loves face packs, and admits having had three in the last week alone. "A good face pack should plump up your skin and get rid of your smaller wrinkles for a few hours," he says. "Never mind Botox and all that rubbish! A nice back rub and a face pack is just as good."

Moisturisers

Homemade moisturisers are nothing like those perfumed unguents you buy in the shops. In order to make those at home, you would need to buy in emulsifiers, which hold oil and water together. The versions you can make at home are a little more basic, and range from mashing up avocado and putting it straight onto your face (tragically, the avocado season is over so I've not been able to try this one out) to letting seaweeds such as bladderwrack, toothrack and kelp simmer in water for 10 minutes, squeezing them out and then using the resulting botanical gel. If you want to get serious about it, it's worth buying cocoa butter online; to make a moisturiser with this Ambrosen warms a few chunks of the stuff on the stove, and then adds a dessert spoon of almond oil, a mash of portobello mushroom and banana, and 3 drops of rose essential oil. We pour it into an ice tray and put in the fridge; when I try it later, I conclude that it is the most amazing moisturiser ever – if you don't mind smelling strongly of chocolate. Which, sadly, I do.

My favourite suggestion, however, is simply putting a handful of oats into some cold water and then leaving them to stand for half an hour. Sieve the oats, giving them a good squeeze as you do, and you are left with the colloidal milk which you apply straight to your skin with cotton wool. Oats have got all sorts of amazing abilities, among which is their talent for calming skin irritations, and when I stood in my kitchen spooning (for your pleasure) oat water on to my face, I swear I really could feel my skin zenning out. Make sure you check for leftover oats, however. Really. Not so zen.

Hair products

One way of improving the appearance of your hair is through what Constantine and Ambrosen call "pre-treats"; a heavy conditioner such as mayonnaise (yes, you can use Hellmann's), or olive oil put on to dry hair and left for 10 minutes or so, before washing with an ordinary shampoo. However, when I obediently shovel Hellmann's on to my hair, it's not only an unpleasant experience, but I am also told later by a friend that my hair looks lank and greasy. I had been quite pleased with the effect. Hmm.

You can dry-clean your hair using cornstarch – just sprinkle it on, and brush through to take out the oil – but Ambrosen admits that it won't be shiny, just clean and a little bit dull. Or you can use eggs. For this you need two eggs, beaten separately and then combined (Constantine claims this makes them less likely to scramble, but can give no good scientific reason for his claim. I am not inclined, however, to test it out). Rub them gently into dry hair and then rinse with very tepid water, for obvious reasons. The protein in the eggs is very salutary, apparently. You can rinse with beer or lemon juice after, or make an infusion of nettles for real shine.

Bath stuff

Things for in the bath? Ambrosen suggests finding a piece of muslin (or any sort of similarly fine material) and putting a couple of tbsns of oats (again!) into it, tying it up and dropping it into the bath as the water is running. "It gives a lovely milkiness to the water," she explains. "You can tip in a spoonful of sodium bicarbonate beforehand to soften the water and deodorise your skin as well."

Into the muslin you might also want to put fresh herbs such as rosemary, sage, bay or thyme, or alternatively you could have a spicy winter bath with a cinnamon stick, some cloves and a whole nutmeg. "Don't just chuck them into the water or they'll turn into what we call knicker-wasps," says Helen. "You know, those herby bath things that you always end up sitting on".

I try it out – sodium bicarb, oats and all – and emerge from the water feeling smooth of skin. I told you: oats rule the world.