I had a situation very similar to yours, dodgy builder using dodgy electrician (and non corgi approved plumbers installing gas boiler).
We fell out with our builder, as you have and we too had one of those off the shelf court letters. I replied, in writing, with a very official letter explaining our grievances and the fact that we may take the situation with the gas and electrics further. The originators of the court letter promptly told us they would not pursue it and we never heard from the builder again.
As regards to electrics, this was more or less finished, but was of course uncertified. Had lots of trouble getting a proper respected guy round to help with this, understandably, but we got one good enough to help us out, checked everything he could, took every socket and fitting off to check, and ran every test he could. Checked the cable runs as best he could with photo's I'd taken during the build.
So we were ok in the end. Your builder hasn't got a leg to stand on, he'll get nowhere, no matter what he threatens you with, and he'll know it.
By the way, one thing our 'electrician' did was cut a cable to a socket too short, so joined it to another in a ball or tape and pushed it out of sight under the plasterboard. Nightmare!