I was just about to reply that I had eliminated that but I thought I'd try again. This time I turned off the hot supply to the downstairs shower (before I thought I'd turned off the mains supply to it) and lo and behold I think the shower is the cause. I also have a water equalising valve...
The last time this question was asked was in 2004, unfortunately the answers have not been any help.
My cold water tank is being filled from the hot water tank. The system has been in place about 15 years. Both tanks are in the loft with the cold water tank raised on a platform. It is an...