Used car trader will only take car back, and not entertain any checks / repairs

[des8]

You requested a repair, but the trader refused and only offered a refund less £200 collection fee.

Well you can opt to have the vehicle repaired via the warranty but:
1) have you checked there a warranty in place
2) if yes to 1) does it cover all the faults
3) will you have to make a contribution to the costs