If you do most of the work yourself you'll take away their profit. They don't like that.
So they won't do it. It's all part of the MC code.
In reality all you want them to do is to connect the gas - 2 hour job at most. £60.
Yes I agree, but I would be happy to pay them double and more than that as i would be getting the job done cheaper still.
Really I just need like you say to find someone that is happy to work with me. I could leave all the pipework exposed so they can check joints etc as the house is currently empty.