Persisitent dropping of Water Pressure on my Baxi Bahama 100 Boolier?

I have just had Baxi carry out a series of repairs to our 10 year old boiler. They were called in under a new warranty taken out for 12 months in March 09, and have needed 5 visits to remove the original problem, which was:- During and after start up the bolier gave out a major 'grinding' noise along with a lowish level 'siren' type noise as it shut down. Throughout the 5 visits, this grinding noise aand behaviour persisted with the Baxi/Heatsave engineers replacing hoses and joints, until the 4th visit, when the engineer spotted that the air prseeure release valve on the top was mis shapen (ovoid) after replacing it, all grinding and siren noises disappeared (? was air in the system the problem all along?)..

Since then the water pressure has persisitently dropped to below 1bar and needed regular 'topping up', but at no time did the bolier fail to operate. Today I noticed that the pipe from boiler to outside drain was dripping consistently and wonder if this is contributing to the persisitent pressure drop as I am unaware of any other leaks in the system Any ideas?