It sounds like that when your water is heating up, it is expanding (as it should do when heated) and spilling outof your open vent into your header tank.
You need to make sure that the vent outlet is min 450mm vertical from the top of the hot water storage tank to allow for expansion.
Try that.
Also, your Central Heating pump may be positioned in the wrong place on your pipework, thus pumping water through through your CH pipework and through your open vent. This could mean that the pump is positioned BEHIND the open vent pipe on your CH pipework (on the direction of flow).
