Header tank overflow on heating system

Hi I have a pumped central heating system, the boiler is a Potterton Netaheat profile 2-4-97. It was apparantly installed approx 1995 after the old one had packed up. The system has a header tank in the loft flowing down through the airing cupboard (their is No 3 Port Valve) and through to the boiler downstairs where the pump is also situated.
Unfortunatly the header tank has started to overflow and also the rad closest to the airing cupboard has stopped heating up (it has been bled). Apparantly it does heat up when just the hot water is turned on ??????
I have replaced the ballvalve in the roof and their is no visable sign of water entering the header tank. After draining the system and refilling it, it begins to overlflow within an hour. Please could anyone advise on how to test for possible faults and how to resolve them.
Thanks

If water is not lfowing form the vent then chances are the coil has gone in the cylinder - so it'll be a new hot water tank.

Which will need the control system upgraded with a thermostat and zone valve.

Happy plumbing!

Also worth checking any mixer taps/shower you have.