OP he

Thanks for all the posts about how to improve flow-through in my kitchen
HW tap. I didn't realize (and certainly was never told by MFI!) that my
fancy tap should really be pressurised! I will be looking at the pump
suggested by a couple of posters, and also at the cheaper alternatives
suggested.

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. What I hadn't considered - as several pointed out -
was that this would increase the amount of cold water waiting to be run
off, before the hot finally came through! duhhh.

Thanks a lot everyone -- very useful exchange, for me.

John