- I don't quite understand why I need the tank - purely because the
boiler
isn't designed to be fed mains-pressure water? All the "unvented
systems"
diagrams I have read up on omit tanks of any description.

Clearly you have not seen diagrams for the correct layout of an unvented
HWC as opposed to boilers with unvented primaries.

Thanks for your advice. Could you just clarify the bit above please? What
exactly is the difference between the 2 setups you mention? What's a
"primary" in this context? (I assume "HWC" is "HW container"?)

Thanks again
Richard