A couple of reporters have been touring British fertility clinics pretending to be infertile and in search of a sperm donor who would help them create a beautiful and intelligent "perfect" baby. In last week's Sunday Times the pair reported, by way of a shocking discovery, that some leading clinics had been happy to oblige, one going so far as to confirm that their sperm-selector was "very picky".

Although those of us fortunate enough to meet the father of our children are allowed to indulge in the lamentably Darwinian habit of mate-pickiness, it is against the rules for the beneficiaries of anonymous donor sperm.

In the wholly man-made world of assisted reproduction, the preference for one type of parent over another is considered quite unnatural. Baroness Warnock, whose report on human fertilisation and embryology led to the HFE Act of 1990, and established her as our ultimate authority on reproductive ethics, duly confirmed that selecting donor characteristics was "against common humanity". She added, according to the Sunday Times, that "If you really want a baby you had better take what's coming".

Although more robustly phrased, this is more or less what her inquiry, established 20 years ago in July 1982, concluded in its final report: "As a matter of principle we do not wish to encourage the possibility of prospective parents seeking donors with specific characteristics by the use of whose semen they hope to give birth to a particular kind of child." And that was that.

In this, as in many other areas, the Warnock-based legislation is now showing its age. If, in practice, persistent couples can now disregard Warnockian precepts on donation etiquette, the law should perhaps be adjusted so that they can be legally treated - and protected from charlatans - within the British system. True, this would mean compromising Warnock's moral position for the sake of practical considerations, but as the philosopher herself argued in 1984, the prevalence rather than the desirability of a treatment may be reason enough to give it the protection of the law: "The practice of AID [artificial insemination by donor] will continue to grow, with or without official sanction and its clandestine practice could be very harmful. It is therefore desirable that AID should be available..."

On the other hand, the law was there to fix boundaries: "There must be some barriers that are not to be crossed, some limits fixed, beyond which people must not be allowed to go," the report said. "Nor is such a wish for containment a mere whim or fancy. The very existence of morality depends on it." For example, there must be no organised surrogacy: "That people should treat others as a means to their own ends, however desirable the consequences, must always be liable to moral objection." Always?

Moral philosophy turns out to be more adaptable, or fallible, than you might think. In her forthcoming book, Making Babies, Baroness Warnock reveals that she has changed her mind. "If surrogacy were allowed in the UK, on the American model, though some people might be offended, I doubt if we would be harmed..."

Showing great honesty, though no obvious regret for any inconvenience caused, the baroness further reveals that she has been reconsidering eligibility for treatment, her inquiry having opined that "it is better for children to be born into a two-parent family, with both father and mother..." Now, Warnock is inclined to think that assisted conception using donors should be available to everybody, "regardless of his/her sexual orientation..." She explains: "It is better to have properly regulated than unregulated assisted conception."

Her most dramatic volte face concerns sperm donors who, contrary to the recommendations of 1984, she now thinks should be able to be identified by their biological children. Her objections had been practical, openness would deter donors, rather than moral.

New evidence - from donors in Scandinavia - has changed her mind. A welcome turnabout, then, but sadly too late to help the thousands of children who have been, and are still being, created in accordance with her earlier view, ignorant of their genetic inheritance.

Warnock's mind is not the only thing to have changed since her inquiry started work. If society's understanding of what constitutes a proper family has been transformed since 1982, the science that informed her findings and the legislation which followed has, as a Commons committee has just reported, been thoroughly superseded, leaving philosophers staggering behind technicians.

While it is right that there should be an attempt to regulate and moralise in this area, the current speed of scientific advances requires that regulation to be regularly re-evaluated, and certainly more often, as Warnock's own divagations suggest, than every 20 years. Everyone opposed to donor anonymity will be glad that she has seen the light, but the very fact that she has done so suggests that fertility and embryology legislation must be a provisional affair.

The current, Warnock-based law cannot offer guidance on - to name just a few practices - ICSI, egg-sharing, assisted reproduction tourism, stem cell research, IVF for post-menopausal women, cloning, and pre-implantation diagnosis, which the inquiry pronounced impossible "for some considerable time".

Now that time has passed, the world has moved on, and both the 1990 act and the philosophy behind it are about as much use, in the complex cases that constantly arise, as the Stephenson's Rocket manual might be in a mobile phone factory. The law, as the Commons science and technology committee has realised, is overdue for revision.

As Warnock said in 1984, "There must be some barriers that are not to be crossed." Or not until a moral philosopher can give you a leg-up.