Thanks for all the replies. You won't believe it but it has started
working properly again as mysteriously as it stopped! However I suspect
it will stop again one of these cold frosty mornings!
From reading the posts I gather the problem is almost certainly with
the 2-port valve. I did try manually opening the valve but the boiler
still would not fire up until I turned on the HW circuit. The valve
always closes quickly with a 'whizz' sound but is quite slow opening
with more of a quiet creak than a whizz. Not sure if this is normal,
sounds like during opening it is tightening a spring loaded coil or
something like that.

Is it tricky to replace the motorised valve? I know how to drain the
system and basic plumbing. Might be a job for my plumber though. Thanks
for all the help which has been very informative.

Cheers,
Chris.