Supply Tank Ballcock Keeps Filling

About every 30mins I can hear the ballcock in the upstairs tank open and allow water to fill the tank. This lasts for about a couple of seconds but is increasing in duration. Is the problem a leak in the Boilermate 2000 and what should I do about it. Thanks for any assistance, Ken.

I think you have answered your own question, but where is the water escaping to? Under the floor? I would look for the scale left by water evaporating, from the heating and boiler pipework. I think you can rule out domestic hot water pipework and taps. Has the boiler been changed for own with a safety valve, is it dripping to waste?
Good luck
Paul Brown.