How do I work out the CH boiler size we should have?
I'm not sure of it's accuracy, but there's a calculator at energysavingtrust.org, and here's a very similar calculator that's a little bit easier to read (imo). I'm guessing the location factor for both of them is based on average winter low temperatures for the region selected. I'd look up climate info on the areas and try to pick something similar to where you live.