In an earlier contribution to this discussion, James
wrote:

Have you thought about dumping the tanks, and fitting a double return
value [which costs a couple of quid] and running the whole system off
mains pressure.

No? Good - *don't * do it!!

Your hot cylinder isn't designed to withstand mains pressure - and doesn't
have all the additional safety features which mains-pressure hot water
systems need.

Mains pressure is fine - provided your mains supply can provide adequate
flow - but you need the right kit, and it has to be installed by someone
with the right qualifications.
--
Cheers,
Set Square