Is it a gledhill thermalstore? Been a pain the last 23 years, so we have just had ours removed and a Baxi combi installed, not liking having to wait for the hot water like. Cost 2.5k with the hive thermastat.The set-up is bizzare.
We have a Potterton Slimline Classic in the kitchen which is where the gas goes.
Then upstairs we have Gledhill Boilermate 2000.
The plumber said the Boilermate is the issue and will continue to be. So he said the best thing to do is get rid of the Boilermate and put a combi in upstairs.