Here's one thing that you might care to consider, Sonyme, if you do decide to get a condensing boiler.
The boiler has to have a condensate pipe fitted, to drain off the condensed vapour from inside the boiler. It's a narrow pipe, about 22mm diameter, but you should make sure that the fitters do not run the pipe so that it drains outside. In extremely cold weather, such as we had last Jan/Feb, the outside part of the pipe can freeze solid, which means that the condensed vapour cannot run away. This can lead to the boiler shutting itself down, until the frozen part of the pipe is thawed out. Try to get the fitters to connect the condensate pipe to the kitchen sink waste pipe, under the sink, thus the entire length of the pipe will be inside the house, and it won't freeze up.