My combi boiler pressure is dropping to below 0.5 bar every day causing the system to show the low pressure warning light, and the system will not fire up.When i increase the pressure to between 1-1.5 bar manually the boiler starts, increases to around 3.5 bar the system then dumps about 600ml of water out of the overflow pipe. When the system stops the pressure drops to below 0.5 bar causing the warning light to come on again. There appears to be no leaks anywhere else. ANY IDEAS (worcester bosch 350 combi 7 years old)