I'm no lawyer, but do you have the original quote? Does that detail the work to be done? Does this tally against what they are trying to charge you? If they are trying to charge you for something you weren't quoted for, why was that work done (and why are you being billed for it if you didn't require/request it)?
Obviously there may be instances where extra work was needed but if they're trying to bill your for unnecessary/unauthorised work thats a good starting point for not paying.