I may have missed it, but I cannot see where the OP might have suggested that there is also a remote header tank, or if there is, where it is located. Helvetia had two tanks, one being a standard Bowman expansion tank on top of the engine, and another at the far end of the boat where the last coolant fed radiatoor was located. If I had fitted a blanking cap to the remote header tank, the heating simply would not work because of restricted circulation, and if I had fitted a blankng cap to the Bowman, I suspect that the heating system tank would have served as the engine pressure relief and probably spewed water out all over the front cabin floor. Or am I missing something?