Nick wrote:
Yes it does get noisier with hot water but is very quite with CH. I
had the gas pressure checked out and its spot on; no problems there.

So could it be just the blocked up CH system as BG claim? If so why
didn't they address that in their annual maintenance programme or at
least tell me about flushing.

I am no expert, but if the CH works, and the DHW if the thing causing the
boiler to trip out, I can't see the problem being with the CH side!

It could be a scaled up heat exchanger on the DHW side, resulting in the
boiler calculating the flow rate and adjusting the burners to what it then
thinks is correct for that situation, but then not getting enough heat away
from the boiler, then the overheat trip triggers.

Do you live in a hard water area, if so, do you have anything to prevent
scale, like an ion exchange water softener?

Toby...