Many thanks to Carl83 for the reply. However, my concern was focussed on the extent to which expansion of the water in the system when hot would cause an overflow into the header tank with the potential for siphonage rather than the pressure as such. The pressure is a factor which I had not considered so thanks for that.
Any overflow and siphonage problems are caused by poor system design specifically in relation to the positioning of the pump and its proximity to the expansion and feed pipe. Any such issues have no connection to the height of the header tank.
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