Hi mate.
Firstly check for the obvious - i.e around the boiler.
check around radiator valves (this is the most common cause)
check joints in pipework (where visable)
check to see if there are any visual damp patches on ceiling areas.
Before taking up flooring a good inspection within the boiler is needed. ( I was losing pressure through a faulty diveter valve which was easily rectified)
If all is lost then it is time to lift floor boards to find either a leaky joint or burst pipe, however it may not come to this.
Good luck. Let us know how you get on. (bet its a rad valve !!!!)
Cheers
Rob