Combi boiler - hot water isn't very hot, but radiators work OK

It's very likely the diverter valve. Switch the CH off, run a hW tap, if the boiler fires and the CH flow pipe gets hot the that's really all the proof you need.

On the assumption that this is not a troll.....The plastic tap you refer to is the control for the Filling Loop to increase the pressure in the boiler .The instructions that were left by the installer ( They were weren't they?) will explain all ( You have read them haven't you ??) .

AIUI the pressure should be read when cold and not hot so it might be that you need to put more water in when it's cold .

Is there not a temp control for the HW so you can increase it to a temp that suits you ?

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.

The most likely failure modes are:

HTH

Thanks Ed,

I ended up getting a gas boiler firm out under a caveat of my home insurance who took a look. After much tutting he decided a whole diverter valve replacement was required. He's due to come tomorrow, lets hope this sorts out my boiler.

Just for completeness, thought i'd post. The diverter valve was replaced and boiler sorted.