Jon Tiffany

In terms of brand, they are all pretty good these days. Baxi, Worcester Bosch and Valliant are all popular brands. Probably what is more important to get the correct size. Some plumbers will just err on the side of caution and spec up a top of the range boiler. With combis the main thing to watch is the flow rate. The Worcester Bosch website has an online find a boiler guide that might give you some ideas.

Central heating pipes are often laid directly into solid floors, normally they are wrapped with insulation, but if they are going into channels in the celotex then no need I guess. I'm not so sure about your plan of adding insulation to the floor - 30mm of celotex and 18mm chipboard - surely that will cause some issues with the floor heights? You are correct in that the current floor most likely has no insulation. How about laying some multifoil insulation down? Much thinnner, but might work out a bit expensive.