You can reasonably argue that they could not sell the car during that period
as it was in the garage having work done ie: panel sprayed, which you are
prepared to pay for, and by you paying for this work you have contributed to the
overall condition of the car making it more salable, and you are agreeable
to them retaining £150 from your deposit for this work.
They may try and bargan with you and then agree £200 in full and final settlement.
Also tell them you will be back to them for a new car soon. Worth a try.