This is a baffling problem! It happened suddenly, not over time. I've replaced the thermostat and safety cut out. I've checked the motorised valve and pump. I've drained the tank to reinstate the air bubble. The water heats up, not as quickly as it used to and supplies reasonable hot water to the bathroom bath...but not the sink and the shower. It's also only luke warm downstairs in the kitchen. After draining the tank the pressure gauge was still reading 2 bar...how can this be? Why should cold water come out of a hot tap? I have 2 mixer showers but these have non return valves in them. This is baffling me...any help gratefully appreciated. 
