Why does my hot water tank get hot when my central heating is on when my hot water control is not on

Is the heating boiler piped to the hot water tank?

Their appears to be no blockage in my central heating yet it continues to vent into my header tank can anyone help

Is the heating boiler piped to the hot water tank?

It is a normal central heating and hot water system.

Not knowing anything about plumbing all I can tell you is that the hot water system works normally but the central heating vents into the header tank as soon as it is switch on

By header tank, do you mean a tank up on or in the floor joists with one pipe going to the boiler? If so, it is perfectly normal for that tank to get warm when the boiler heats.