BOILERMATE 2000 constant overflow

Hello,
I have a problem with my boilermate 2000 system where the header tank is continually overflowing resulting in water constantly coming out of the overflow pipe situated over my intergrated garage roof. It has become such a problem that the water is starting to ingress and soak the cavity wall insulation. I've had local plumbers round but they have just scratched their heads suggesting either a sagging plastic header tank meaning the ballcock valve is not seated properly to a blockage in the system somewhere and the system overpressurising. Both pipes into the header tank are warm which could be a clue. I have no problems with my central heating or hot water. Can you suggest what might be wrong?

Turn the ball valve off.
Take some water out of the small tank.
And see what happens.
Mike