I've just had a quote to replace a boiler. £1500 +VAT. I guess that's
the going rate around here in London, although it looks like an
incredibly straightforward job. There's plenty of room to work and so on.

My question relates to the boiler they are suggesting: "Supply and fit
an Ideal Combi 25 KW Boiler".

What would the assembled minds here suggest is a good make/model of
boiler to install?

Presumably a 25KW combi is required, but may go up to 28 KW, as we are
thinking of converting the garage into an extra room. At the moment it's
a 2 bed bungalow without very good insulation. Or is 24/25 KW plenty for
heating, regardless, and the size is determined more by the need to
provide a decent supply of hot water?