Actually finding one with vacancies can be difficult. DS1 has a tutor for English, she was recommended by a colleague who has an older son. He was predicted a C and with the benefit of tutoring he got A*. That seemed like a good reference to me. DS find the tutor approaches the subject differently from his teacher and this seems to work as they complement each other. We take him to her house (20 mile round trip as we live in the sticks) and he does 1 hour a week, plus she sets homework.
I am trying to find a GCSE science tutor...one agency haven't had the decency to reply and the other has tried to set me up with 2 totally inappropriate people. I think personal recommendation is the way to go.