loosing pressure?

had a betacom 30 installed last nov.plumber used a mixture of plastic and copper pipe on the rad cicuits.
due to the nature of the instalation some of the pipework is under celotex insulation which in turn is coverered with a new cement floor.just noticed that the boiler is loosing about 0.5bar every 4-5 weeks.
i dont want to start digging up my new floor just yet and was considering adding a leak sealent into the system,however my local plumbers merchent said it could drasticley reduce the life of a combi boiler.
have checked all the rad valves and elbows by the rads.please dont tell me the floors gotta come up!