Every two weeks or so I have to go and top up the system because the hot taps are running cold. Boiler display is flashing 0.6 bar and I fill up to 1.3. I've had an engineer look inside the boiler and he can't see anything wrong. I've checked the pipes all over the house and cannot see any obvious leaks. Is it worth replacing the radiator valves? They aren't that old and it seems to be quite a lot to be losing all the time for it to be a small leak but I can't think what else it could be.