Chris Bacon wrote:

What is the "header tank"?

I'm referring to the main tank in the loft that is filled with cold
water direct from the supply which I assume is also used as a source
for flushing loo's and things?

You've got a blockage.

I'd suspected this might be the case I think it's a fully
circulating system so as the radiators have been working fine (and
water flowing round) does this indicate the blockage in immediately
local to the cylinder or in the cylinder itself? Is there an easy way
to check/clear it?

Thanks so much for your help!

CS.