Disputing garage bill without going to Court?

The answer would be no there is little you can do since you have kinda let yourself be at their mercy its that old saying of closing the door once the horse has bolted the only thing you can do is not let them do the work if the price is too high but if I get what your saying right the garage has already completed some work you will have to pay for this as you were not forced but chose to leave your car with the garage you need to get a written quotation from them on any more work that they say needs doing but not agree to it until you have more quotes for price comparison