Kalico wrote:

Not sure, but getting a 22mm pipe for the hot as near as possible to
the shower, with other services branching off in 15mm, might make a
difference.


That would mean twice as much water sitting in the pipe between the
boiler and the shower. If this is also used to feed a basin tap then
anyone wanting a bit of warm water to rinse their hands will have to run
off twice as much water.

IanC