Low pressure on combi boiler?

I recently took a radiator off the wall to decorate. since then we have lost pressure in the boiler on a regualr basis. We replaced the filler loop so we could put pressure back into the system, but now we have a leak from the boiler to an over flow pipe that runs out side of the house. we have measured the amount of water we are losing and its about 1 cup full over a period of about 7-10 days. Therefore we need to put pressure back in, every week or so. Is there anything we could do?? (We have flushed the system using the 3 bar knob, to try to dislodge any dirt/grime several times, but although this has reduced the leak, it hasnt solved it!

Any ideas!?