Thanks for all the replies, I feel like I've poked a hornets nest!
Anyway, I managed to get a couple of plumbers out yesterday to look at the problem. First one told me it was the tank at fault and I'd need a new one to start with. Then they talked about changing the boiler to a combi as it would be tons better etc. Second one arrived and first thing he looked at was the position valve actuator which several of you have mentioned. He removed it and checked the valve which wasn't stuck and then put it back on. Turned the water/heating off/on a couple of times and the switch that moves from one to the other was sticking so said I'd need a new one so coming back on Monday to fit it (the first guy never even looked at the actuator either.). Since then however my water has been red hot again. Gonna stick with my current setup as well until it's past it's time. Mrs saw the silver lining though, said as we're now not forking out for a new boiler we can have a holiday with the money we've saved
Anyway, I managed to get a couple of plumbers out yesterday to look at the problem. First one told me it was the tank at fault and I'd need a new one to start with. Then they talked about changing the boiler to a combi as it would be tons better etc. Second one arrived and first thing he looked at was the position valve actuator which several of you have mentioned. He removed it and checked the valve which wasn't stuck and then put it back on. Turned the water/heating off/on a couple of times and the switch that moves from one to the other was sticking so said I'd need a new one so coming back on Monday to fit it (the first guy never even looked at the actuator either.). Since then however my water has been red hot again. Gonna stick with my current setup as well until it's past it's time. Mrs saw the silver lining though, said as we're now not forking out for a new boiler we can have a holiday with the money we've saved