Clarity at last! Many unis have scoring systems for choosing PhD students but this is the only published one that I have found ....
www.sos.bangor.ac.uk/research/docs/phd_studentship_nominations.htm

This scoring system gives you a fair idea of the sort of applications people are looking for. Remember that this is not a standard system, just one used by one department in one institution and that in most cases a good interview performance will make most scoring irrelevant.

Thank you marinegirl!! It does make constant failure a little clearer!

People in other industries have told me that the selection committee are legally bound to explaining to each candidate why they were unsuccessful. Is this true? And is it possible to ask without irritating people that I might want to apply to again in the future?