Baxi combi 105 losing pressure?

Hi
I have a baxi combi 105, about 5 years old. Just recently it has started to lose pressure and needs the system recharging after less than a days use.
There is no water coming out of the pressure relief pipe and there are no leaks at the radiators. I have tried looking for leaks under the floorboards with a camera (not enough room to get uner) but cant see anything. I have bled the system of any air.
Is there anyway of confirming that the problem is definately a leak to the outside (as opposed to an internal leak at the boiler?) before i start taking up the floors. All the pipe joints under the floor are soldered, there are no screw fittings.
Is it worthwhile using some leak fix or is this just a waste of time.

Gary
November 2014