as far as i can remember, if you have let him do repairs, this is as good as saying you have accepted it, but regardless of this if he is a dealer then you should have a three month warranty on parts, which means he should fix it to your satisfaction,
This is why a lot of dealers advertise in their local papers as private sellers, I don't know if this is the case here, but if it is, and he insists he is selling privately, then just remind him that he as a dealer has obligations to you the customer,
If you had taken it straight back, and refused repair, you could have used the not fit for purpose clause.
hope this has been of help.
good luck,
pete.