they can stop you taking it back if you refuse to pay. offer to pay something to cover the work your happy with then take it. if they phone the police the police wont stop you from taking the car back as it then becomes a civil dispute over amount outstanding rather than simple non payment.
the garage owner then has to persue you through the courts.in fact it can be better if you phone the police yourself asking them to attend as you fear the garage may turn nasty.
then once you have your car back burn the place down.