Thank you very much buck51. It turns out you were right and I eventually traced the fault due to an open valve pressurising the balanced circuit (strange plumbing!) and back pressure through thermostatic mixer.
All sorted now.
Giles
Thanks. I do have thermostatic mixer valves but they connected to the balanced cold which is supplied by the same pressure reducing valve. It is a thought through and I will explore what other systems are supplied from the unbalanced mains.
Thanks again and any other ideas.
Am having an problem with my sealed HW system pressure.
Recently I noticed water dripping through the tundish from the relief valve on the cyclinder. What seems to be happening is that the pressure in the HW cylinder rises to 6-7 bar and the pressure relief valve (7 bar) starts to drip/vent...