Low pressure on new boiler?

Had a new boiler fitted last monday, it's a Glow worm flexicom 18sxi. Everything was working fine till that evening, then the pressure dropped, boiler reading 0.4 bar, called someone out and they topped up the pressure. This kept happening all the time, so had new radiators, pipes hot and cold water tanks fitted yesterday, "apparently" this kept happening because there was a slow leak, even though there was no visible sign of a leak, could of been under the floorboards. All was fine until this afternoon when the same problem happened again, called the council out and all what they could do was top up the pressure till Monday. But not long after they left the pressure dropped and the boiler read fault F9. Now i really don't understand what is happening, the person from the council who came up today said there could be a slow leak under the hot water tank, or pipes, but all the pipes and radiators are visible and there is no sign of water. I've been looking around on the net and there's many people in the same situation and there response it maybe there's a leak or air lock in the boiler. As I can't call the people out who fitted it till Monday i just wanted to know if there's anyone out there who can give me some idea of what it sounds like to them, as it's a mystery to me! Thanks