Reading your post further, I now see the deed is already done... so... without any delay, and I mean right now, sit down and compose a carefully worded history of what's occurred... from start to finish. Did you seek out the "contractor", when, where and how. When and what were the details of any discussions? Include all the details. Having learned a hard lesson in almost exactly the same circumstances and wathcing the same story unfold in other cases, I know, that at least under American law and customs, th eperson who shows up at the inevitable court hearing well prepared with written history stands a much better chance of prevailing. Additionally, I, personally wouldn't pay the "contractor" another dime (or the British equivalent) until he agrees, in writing to a settlement. Best of luck, but, unless I'm mistaken (hopefully) the fat's in the fire, so to speak...
Again, best of luck!