Hi, I'm new to this but really need some help. I have a traditional central heating system with hot water cylinder in airing cupboard, tank just above and one in the loft, and boiler in kitchen. The system is such that you can have water only but to have heating you also need the water on. The Boiler is a British Gas 330 installed 7 years ago at which point it had all new rads and was powerflushed. Since the annual service in early August it took much longer to heat water and the boiler would cycle on and off every two seconds or so. In the second week of September it switched off mid cycle with F5 overheat fault. Two british gas visits later, one which failed to find an issue, they replaced the thermostatic valve linked to the tank, said all fine having reset the code and left. Code came again next time used but also caused a leak in a joint which had never been there before. This was fixed, bypas valve opened wider, boiler reset again. Next time system back on f5code again after about an hour. Its five visits now, getting very annoyed now. Any suggestions what could be wrong?