Just for info: I'd thought about increasing the diameter of the
feed-pipe to 22mm because I was thinking in terms of weight of water
coming to the tap.

Which might have some merit if the pressure created by a head of water was
actually affected by the pipe diameter - which it isn't.


That's correct for a static head.
However, under flowing conditions, the pressure drop over the length of pipe
will be less with the larger pipe.

( At the cost of a larger volume to be flushed befor the hot arrives. )

Not that it makes the slightest difference in this case.. the pressure drop
in the pipe is insignificant compared to the pressure difference between a
( say ) 3 Bar pressurised system required here and a fractional bar gravity
fed system which is what we have. Even replacing it with the fattest, most
directly-routed pipe imaginable can still only bring the flowing pressure
closer to the static head, which itself is still way to small.

--
Ron