If you judge this situation by the adage that your gut initial impressions are the right ones, I would get out of this deal NOW. You have a contract, which has already been violated. What's next?
This is NOT going to get any better, IMHO, and I would be on the phone right now asking who authorized a debit of £420, when £350 was the agreed amount. If you aren't happy with the answer, get the money back now, and start over. Hard as that is, it will probably be the best thing you can do.
I usually start these conversations with 'help me to understand why x, y, z happened, when you said a,b,c' ..... that puts the onus on them to start EXPLAINING, and you aren't seen as the aggressor.
Good luck.