GlowWorm 18hxi intermittent hot water problem

[jaytc2003]

Hi I have the glowworm 18hxi combi boiler and occasionally (more frequently) we get only lukewarm water when we run the hot taps etc. This seems to happen more frequently if we have the ch turned up (ch is always on at about 18c sometimes if it is cold we turn it up)

The boiler runs 12 rads, 8 of which are regular size and 4 are a smaller size, central heating appears fine as all rads heat up when required etc. The pressure in the system is within spec as well, and the boiler fires up when needed and doesnt show any errors.
Water pressure from the taps seems fine as well so I dont think there is any blockage

Anyone any ideas on what the problem could be?

Cheers
James