worcester 24I junior losing pressure?

I have a static caravan with a Worcester 24I Junior boiler. The pressure gauge goes down slowly over a period of 3 weeks. I have had the Bosch engineer out twice. First time he increased air pressure from .5 upto 1 Bar. When he left the water pressure gauge went down overnight. Topped it up and it went down overnight again. Second visit from Bosch engineer, he rang colleague who said the air pressure should be .5 as a caravan works different to a Home boiler. He then put the air pressure back to .5 Bar and set water pressure gauge to 1.5 bar. The water pressure gauge is now back to going down slowly over 3 weeks. I have turned of the valves from the boiler to the CH system (the pipes on the far left and right) unplugger the boiler and left for 10 days. The needle still goes down which tends to suggest the problem is with the boiler. I can't feel any water on the pipes to the boiler although there are some traces of green on the pipes. The gauge seems to go down quicker when the Central Heating is being used than when only hot water is being used. The decrease is not apparent during the day when we are using the hot water but has always decreased when I look first thing on a morning. Can't see any visible signs of leaks at radiators or outside pipeworks. Radiators seem to be working at correct temperatures.

Any suggestions please.