Assuming that there was nothing contradicting this on the form that you signed, the agreement was that, if you weren't happy with the repairs, you could get your deposit back so the garage isn't obliged to refund your deposit until they've had a chance to do the repairs and show them to you.
You can contact them now and see what they say or you can wait until they've done the repairs. If they get the impression that you are messing them around, they may refuse to refund the deposit which would mean that you might have to resort to legal action. In general, even the most reasonable garage will consider being awkward over something like this if they feel that the customer is being awkward.
You can certainly walk away from the deal (it's best to let the garage know) but my opinion is that you should consider the deposit as lost and, if you can manage to get it back, that would be a bonus.