On Sunday, 27 November 2016 23:08:07 UTC, MrCheerful wrote:
On 27/11/2016 23:02, Fredxxx wrote:
On 27/11/2016 22:23, MrCheerful wrote:
On 27/11/2016 21:59, Fredxxx wrote:
On 27/11/2016 21:45, David wrote:

snip

Another alternative, if the boiler is feeding an indirect hot water
cylinder, is a leak from primary to secondary within the cylinder.
This would be difficult to spot unless the leakage rate is high
enough to cause the header tank to overflow, which would happen only
if none of the taps in the house are run for an extended period...


snip it is an unlikely scenario IMO.

A PRV, pressure vessel leak, air release valve or rad or pipe leak, in
that order seem much higher possibilities. Particularly if there is
underfloor heating then :a pipe leak.

I agree entirely. From the sound of it, though, the OP (and British Gas) are well on the way to eliminating the likely scenarios, so I thought I'd chip in with something more unlikely