Thanks Ed and Stuart,

Ed - sounds like it could be that, although when I turn the hot tap
on, there is immediately a burst of hot water, then it goes lukewarm
at best for sustained periods of time.

Stuart - Why would I be a troll? I have a problem with my boiler, so
I posted to a diy usenet group, please explain why you think I may
have been a troll?

I have put more water in the boiler from cold though to raise the
pressure to 1.2 bar, this seemed to work for a bit then after a day or
so I'm left with the lukewarm-at-best problem as described.