Andy Hall wrote:

In this case, bigger is better.

Combi boilers are specified in terms of the rate of flow that they can
achieve for a given temperature rise - usually 35 degrees, but do
check specs.

I have a hard time to beat the Worcester Bosch 40 Greenstar with a
flow rate of 16l/min. Baxi, Vaillant and the lot do not seem to match
even this. Where can I find bigger flow rated boilers then?

For almost all with any significant power level, an upgrade to 22mm or
above of the gas pipe from the meter to the appliance is needed if it
hasn't been done already.

That seems to be the right choice anyhow. Too bad that in my case that
is going to be necessary. At least that is what the plumber said and
he seems trustworthy. Speaking of trustworthy, how do you gauge a
plumber? I go by gut feeling and things like this: warnings that the
pipe needs upgrading and so on, and what are the implications of a
condensing boiler yadayada. I am not savvy enough to ask him much
more. As long as he can beat a British Gas quote, I am usually not
concerned about money too much anymore. Is that about right?

Fred