NT wrote:

1. Is a combination cylinder + header tank. That would work, but
it's not ideal as the pressure would be quite low. OTOH, as you say,
there are no safety issues. Where do I look for a secondhand one of
these?

any plumber / heating engineer / scrap dealer / ebay / a local company
could keep the next one for you etc

That was quick! Thanks.


2. Is an instantaneous electric water heater. That's beginning to
look like the simplest and cheapest solution. Do you know if there
are any special regulations for fitting these, as there are for
Megaflos and the like?

the issue is the current draw. To fill a bath at sensible speed would
take a whole lot of power.

Yes. I might have to remove the bath and fit a shower.

Thanks to everyone again.