Advice not trolling please, appreciate i may not have chosen the right builder but they were polished at the outset and gave us a good feeling, you live and learn.
We had a similar issue with our loft conversion. In our case, it became clear that the builder wasn’t going to finish the job (they barely turned up for about two months) and we had numerous concerns about the quality of the work, so we gave them notice and chose a point in the payment schedule where we lost the least. They went into liquidation shortly afterwards.
We managed to get hold of a couple of the sub contractors directly to do the staircase and electrics, we then employed another general building firm to finish the fit out – bathroom, tiling, heating, fire doors, plastering, decorating. A roofer was employed directly to sort out the mess left by the original firm.
We’re in Bristol too: I can recommend http://www.sgwbuildersbristol.co.uk/
They did the fit out, and to a good standard. We had one issue (leaking shower) which they sorted out quickly.
As for building regs, we spoke to the council, they inspected it, and signed it off. We had the electrical certificate from the electrician and I think the council inspector had already seen the steels go in, earlier in the job.