Try personal-tutors.co.uk, they are a national agency (not certain if they cover Wales, worth trying). I happen to be a tutor, although nowhere near Cardiff; I get a steady flow of work from them, they seem to have a good reputation, they have been around a long time.
I need a GCSE Physics Tutor in the Cardiff area. I've looked at some websites offering tutors but am a bit reluctant to register with any of them. Has anyone got any good or bad experiences to share - either of individual Physics tutors in this area, or of the tutoring websites (first tutors, the tutor pages, personal tutors etc etc ...)