Hi stickinsect,
I'm looking for PhDs too, but have spoken to friends doing PhDs, and also ex-lecturers. In no particular order, I've been told to consider:
1. The reputation of your potential supervisor. You could see how often he/she publishes, and in which journals. Is he/she an editor of any journals?
2. The reputation of the department and, to a lesser extent, the uni, for research, especialy in your area.
3. The precise subject of the thesis and, possibly more importantly, the techniques in which you will become trained.
4. The town/city in which the university is situated. If you really don't like the place you study, you may well not be happy there.
5. Does the department have any links with other universities or industry?
I think those five are probably the main ones, although I'm sure other people will come up with more