I have now solved my three dead legs by putting valves on the end and
then bleeding air. (Thanks for the tip!) The combi boilder now does
not fire when I use the cold water system.

However, the boiler still seems to fire for a couple of seconds every
so often (haven't timed it but it seems to be about every hour).

My only explanation is that this is the pre-heat function keeping a
little reservoir of water warm. Am I correct? If so, does anyone know
how to turn this functionality off on a Glow-worm CXI combi??

Thanks

J