go down to take it back with the amount you think its worth and if he calls the police they wont stop you as you are not refusing to pay just disputing the amount or you could always report it stolen the give the police an 'annonomous' tip off about its whereabouts and deny ever taking it there in the first place as like you said there is nothing on paper just verbal agreements.then heartily laugh as not only do you get your car back free but the garage owner gets carted off to the cells when i had my problems with my engine conversion the trading standards people advised me that although they agreed with me about my problems it would be better to bare with it and try to resolve it ammicably. they offered to send an official complaint letter to the garage owner on my behalf and advised me to note all conversations and correspondance for any possible court case but i gave them the benefit of the doubt and didnt make it official and ended up getting my car back finished the next week anyway only 4 months late. the time quoted by your garage is not a reasonable time scale for the work being done