Anyway. You must take some positive steps to get this sorted and move on. Give the builder a list of outstanding items and a reasonable timescale to get them sorted. Tell him you will pay him in full when the works are done and finished to a satisfactory standard. Tell him if he does not complete the works you will use the money to pay another builder to do so.
Update: We moved in a few weeks ago. I have since discovered that my extension roof has been very poorly felted, that the bathroom leaks, that the programmer for the boiler has been incorrectly wired, and various other odds and ends, all of which vindicate my decision to withhold the money.
Builder now says he will not finish off the works unless I give him some payment in advance 'as a goodwill gesture.'
At the same time, he does not let me bring anyone else in to finish off his work.
I may be between a rock and a hard place!