Hi guys,
Me again!
Just thought I'd keep this post updated in case anyone reads this thread and has similar problems.
Had the installer in today and we've ruled out blockages in the pipe work and the radiators being piped wrongly.
If you remember - the boiler heats the hot water really well (burner on max and flow temp at 70 degrees) but when the demand for hot water was satisfied it started hearing the rads but flow temperature would start at 70 degrees and then lower until it hovered around 45 degrees. This happened even though we disconnected the Internet (so no weather compensator data used), the room stat in the fridge (3 degrees) and 'comfort override' mode on.
We then tricked the boiler when it was heating hot water to heat the rads instead (at 70 degrees with burner on max) by manually opening the motorised zone valve. All of the radiators (bar a couple that had dodgy valves which we've now fixed) and all the rads were roasting. So the pipe work and rads are fine.
So the installer thinks it might be the controls. However Vaillant have come out twice and not identified the problem. I think it might be the fact that the boiler is oversized (30kw but have 14 rads over 4 floors plus unvented cylinder) so it might be in anti-cycling mode? What do you think?