Seems reasonable to me. PSTT lose the test fee if you don’t show up, regardless of the reason. Part of the cost to you is the use of the vehicle and fuel for the test. You didn’t do the test, and PSTT lost use of the vehicle for anyone else, while still offering to train you later when you’re better. You used two time slots; but they only get one set of fees.
I trained with them and found them to be excellent. If you go anywhere else I’ll bet it will cost more than £350! It ■■■■■, yes, but you’ve already started, so might as well finish there, too.