We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "John"
saying something like:

Thanks for all so far. Thinking about it, there is a mixer tap in the
basement kitchen but I can't see mains water pushing back the HW against
the head it has down there. I can't remember if the bath on the ground
floor has a mixer but it's certainly not a bath/shower mixer. The shower
itself is pumped off the storage tank.


If the MCWS head is adequate to push up to the storage tank above its enough
to create your problem John

I had exactly this backfeed problem when fitting a mixer tap (French).
It mixed in the body and not at the exit. It didn't get to the stage of
overflowing the storage tank, I sussed it out immediately when the flow
of hot was freezing cold immediately after turning off the cold tap, as
the introduced cold water in the hot pipe made its way back down.
--

Dave