John Rumm wrote on 26/03/2018 :
I have not checked, but I would be surprised if the gas inlet is 22mm on the
boiler - so it will have to reduce somewhere!

What other appliances are fed from the same pipe run?

A 3.5Kw radiant gas fire and a gas oven/ hob, both branched directly
off the 22mm run, then the final section of pipe at 15mm feeding the
boiler. All as installed originally, except back then, the original
boiler was a 32Kw unit and replaced 10 years ago with an 18kw
condensing boiler, because much less heat input was needed.

The installer fitting the last boiler, suggested it would need a new
pipe installed, on the surface - a long and ugly route around much of
the the outside of the house. My pipe calcs at the time suggested that
was rubbish and so it proved, because he gave in and used the existing
supply which has been absolutely fine for the past 10 years.

Now another installer is suggesting the existing may not be adequate,
from my description over the phone and is suggesting Worcester
guarantee would be invalidated if the final 15mm pipe were not
upgraded. Worcester said, if the pressure was adequate at the boiler,
with all in use, then no problem at all.