Chris McBrien wrote:
I my bathroom and shower are
right next to and above the IMMERSION cylinder so that's where the main hot
water usage is. The kitchen sink is at the other end of the house and tends
to use the hot/warm water from the FIRE cylinder as it is closer.

If this is so, it suggests that you have the two cylinders connected in
parrallel. If so, when you turn on a hot tap, you will get some water
from both cylinders simultaneously. If one is hot and the other cold,
this will be a problem.


The DHWS pipework from the 2 cylinders should be connected in series,
see above. All the hot water supplied should be coming out of the top
outlet of the FIRE cylinder

I've looked at the BES website mentioned above and because the sring valve
is so cheap I'm going to give that a try first. If it fails I'll go back to
plan (a) and fit the zone valve.

You shouldn't have any valves in the open vent pipe from the top of the
cylinders, zone valves and check valves included.