Set Square wrote:
And it's the larger tank which continues to fill, and overflows - even with
the water supply turned off?

Then I'm flummoxed!

The suggestion made by someone regarding some sort of cross-coupling between
the mains cold water system and low pressure hot water system seems worthy
of investigation.

Downstairs we have a gravity fed mixer shower. Water comes direct from
the hot water tank into one side, and from the mains feed into another.
The temperature control is separate from the on-off control. Normally we
keep the temparature near maximum hot. Overnight last night as a test I
set it to cold...and this morning the level in the feeder tank hadn't
risen a millimetre.

This looks like it could be my chief suspect although quite exactly what
is happening I'm not sure.

Thanks,

Andrew