In article , Nick Brooks nick.brooks_N
writes
Here is a schematic of what I propose. Please let me hace any suggestions.
http://www.saucefx.com/nick/heating/heating_system.jpg
many thanks
Nick

The expansion vessel for the DHW is to accommodate expansion of the
water in the cylinder when it is heated and so needs to have an
unrestricted connection to the cylinder; you've currently got a non return
valve between them.

Be aware that UK water regs prohibit amateur installation of unvented
DHW systems and that building regs approval is required.
--
fred