combi not holding pressure

Mine does this once a year or so as well.

I just repressurise it and wait another year.

There's a distinct possibility that you may have a problem with the pressure vessel. If this loses its air - needing re-charging - or fails completely, the water in the system has no 'elbow room' in which to expand when its gets hot. The result is that you pressurise the system cold, turn it on so that it gets hot, the water expands and the pressure rises above 3 bar - causing the pressure relief valve to operate. The system cools down again and, because water has been lost through the relief valve, the pressure falls to zero or near to zero.

The pressure relief valve will discharge through an outside wall. You need to find the pipe, and hang a tin under it to collect any water which comes out. Then re-pressurise your system cold and run it again - and see whether any water collects in the tin. If it does (other than rainwater!), my suggested diagnosis is correct. If this is the case, come back - and we'll tell you what to do next.

The manual valves which are situated infront and behind the pump (water) are buggers for leaking, and are hard to spot, they leak when cold and seal when hot, the rubber washer in them perishes. both mine are weeping slightly, when I get around to it I'll pull em and solder them up, it only means a system drain if I need to replace the pump.

check out the SealedCH FAQ.