MatSav wrote:

Could be that your boiler thermostat is kaputt, and the "flow" is
boiling, passing a mix of steam and water up the expansion pipe into
the header tank. A tell-tale sign is "hammering" noises in the
pipework. Chances are that the over-pressure may also have damaged the
impeller of your pump - the sign of this is very hot radiators
upstairs, but cold downstairs.

Hmm there are no hammering noises although the upstairs rads could be
a hotter than those downstairs but they are not stone cold. Couldn't
it just mean the system needs balancing?

Thanks for replying

Kind regards

Ross