get some quotes from some reputable garages to compare the costs and then offer to pay that...hoping its cheaper of course!
Unfortunately it will be their word against yours in court and the mere fact you let them pick the car up does tend to swing in their favour. I.e. they pick the car up, offer a quote, you think the quote is too expensive so then what??...they then drop the car back to you? Im not saying you agreed to them doing the work before hand...I'm just saying what I think it tends to appear like. Hope it turns out ok in the end.