If the header tank is constantly taking on water there's a leak alright. Can you isolate the ground floor circuit ( any underground pipe) and see if the circuit fills then.
If ye can isolate the ground floor totally, try putting mains water through it. This will increase the noise of the leak flushing out. You could use a listening stick which is basically a 3-4ft long steel pole with a wooden handle on top. (I know it sounds mad)
Or use a long screwdriver, place point on rad valves/visible bits of pipe etc. you should hear water noise flowing. When noise gets louder your getting closer. Failing that there are companies out there that specialise in leak finding.
If the leak is only slow and the gr floor heat works you could try running your hand around floor to see if the leak is heating the tiled floor.
Check for damp patches in walls/skirting boards warping slightly. Even a leaking rad valve dripping down to the pipes under d floor rotting them.
It's trial and error I'm afraid, good luck