Thanks to all who made suggestions about my problem of not having equal
or adequate pressures to an all-in-one power shower.

In the end I took a tee off the hot as it came out of the cylinder, ran
it up to the loft in 22mm with a full bore gate valve for isolation,
round a couple of swept bends then down 15mm to the shower, the same as
for the cold from the header tank.

It works a treat now - pressures during the swarf clearing bare-pipe
stage were identical, and I can run the pump at full blast without
starving it.

Quite a bit more work than the 'replace like-for-like' that I started
with, but satisfying to get it done.

Also quite scary to do it at a friends house while she was away for a few
days, then leave it unmonitored until she came back. I *thought* I'd
managed it with no leaks, but it was heating/cooling all the time, and
inside a wall etc. I had visions of her returning and ringing to ask me
what I planned to do about her new indoor water feature...