"Christian McArdle" writes:

So, if I fit the non-return valve, does this mean that when running
the tap hard the vacuume created by the downward water to my tap will
effectively provide a suction force on my hot water cyclinder, thus
providing sufficient flow to the tap.

Yes. It may also explode, killing you, your family and your neighbours.

Sorry Chritian,

Did I miss something here? So its not a good idea to fit a non-return
valve on the vent? When you said explode, did you mean implode? I don't
like the bit about killing people and stuff.

The input pipe from cold tank to water cyclinder is already 28mm,
but the cold water tank is only slightly above (couple of foot)
above cylinder. But it's not possible to move the cylinder down
a floor (no room!).

Cheers!
Colin.