Ian,
I don't recall fatigue EVER being cited as a causal factor, it is only ever cited as a contributory factor. The fact that two relatively bright guys end up doing something rather dumb on a dark night after 13 hours of duty is popularly called human error. The fact that those guys were acting on only 25% of their normal capacity due fatigue was only a contributory cause.
We are not yet sufficiently enlightened to appreciate the folly of pilots working these kind of durations. For the AAIB to make a recommendation for change to CAP371 would take an accident of such size and controversy where fatigue played a primary role.
We have seen prosecutions recently of drivers working professionally in unregulated situations where they were found culpable after accidents. The law has already decided that being tired amounts to recklessness and has delivered several judgements accordingly. Perhaps you might ponder the wisdom of flying an aircraft of infinitely greater complexity towards the end of a duty period of 12/13/14 hours, after all the most demanding phase is always at the end of a flight!
CAP 371 was designed as a sensible maximum in the age when rostering officers used pencil and paper. Today trip sequences are constructed by computers to achieve the greatest possible productivity within the regulatory confines of the maximum legal day. Working to the absolute limit has become the industry norm.
Do not make the mistake of thinking our regulatory body, the CAA, is the guardian of air safety in the United Kingdom. De-Facto... the CAA exists to protect the commercial well-being of UK airlines. For the CAA to legislate in a manner that might place a UK airline at a commercial disadvantage to a European competitor would be utterly unheard of.