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...

Why would running a tap make any difference?