achrn, I agree with Alec1.
your builder is the man you need to direct your thoughts at. its him who pick the Muppet and it will be him that needs to get it sorted.
That's what I've been doing all along and am still doing - with the one exception of at the point that when the plumber had connected up the cylinder with no expansion vessel effective and said it was a complete and safe install I instructed that he was not to touch anything else in my house.
But there are those here saying 'it all went downhill when you let the builder choose the plumber rather than taking control yourself' and so on - which I took as being a view that I should be picking the plumber, and therefore should now pick the plumber to get it fixed.
The current arrangement with the builder is:
The builder is getting in a plumber of his choosing to do the washbasin sink taps, toilet soil pipe connections, etc - all the true 'plumbing' stuff that remains.
The builder is getting someone he uses occasionally who describes himself as a heating engineer, is Worcester Bosch accredited, and definitely has Gas Safe unvented on his ID card in to look at the heating install. (The builder doesn't use him much because he won't connect toilets.)
If this chap says anything needs changing then it will be changed. The builder is covering all of this within his already agreed fee.
Before any changes are made to current system, I am getting in someone who I checked was Gas Safe for pipework and boilers and unvented cylinders before I even picked up the phone. He is going to inspect all the heating system and gas work and give me his advice. He has not done work for this builder. He is appointed by me and I am paying him. hopefully this chap and teh builders new chap will be in agreement about what is needed.
Is this arrangement where there are two people doing different parts of plumbing (one 'plumbing plumbing' and one 'heating system') a sensible approach? It was alluded to in passing earlier in the thread as being a good way forward, but do I need to beware of any pitfalls that way? (Other than synchronising work, which we are all agreed is the builder's problem).