John Rumm has brought this to us :
I have noted in the past that many installers seem to work to a rule of thumb
when it comes to pipe sizing rather than actually doing the sums. So if they
always suggest upgrading the pipe, then it will usually be more than adequate
without needing to think about it.

A guy called earlier, to look at the job and he agreed with me it was
fine as is, unless a large combi were to be installed. The 22mm pipe
run at a guess, is around 13 to 15metres, before it drops down to 15mm
for the final bit to the boiler.