Not quite getting this, when you say you left the car in the garage over night with the "handbrake down" are you saying the handbrake was not on and that the brakes had stuck on by themselves?
I agree with the comment about not using the handbrake and leaving in gear, a bit perplexed about the brakes still locking though, if you are using the hand brake I can understand they could stick but as said they are seperate to your disc brakes.
£700 they didnt half screw you, do not go back to them again, yes make an official complaint. If you stick your location on maybe a member on here could either advise you on a decent garage or pop round and have a look at it. let us know what happens.