Header tank expansion pipe has constant flow of water coming from it even when boiler/pump off.

Your heating circuit is partially blocked with sludge/debris, suggest you invite a plumber/gas filter to quote for power flush. As water cannot easily move around your heating circuit due to the blockage it creates back pressure and finally discharges into your F&E tank. Sludge build up in open vent systems is common, but could have been avoided if routine maintenance was carried out. Also would suggest you have a magnetic filter fitted such as Fernox TF1 filter. Once your system is power flushed and clean, the filter will help prevent a re-occurrence of this. Average cost of power flush on a 10 to 12 rad system £250 to £300 inc all labour and chemicals. Once the system is clean ask the operative for a power flush certificate. In addition to the previous post by Kevin Cassidy, I believe he may be mistaking your cold water storage tank for the heating feed & expansion tank, as you stated in your original post "I have water coming from my expansion pipe over the header tank" Please clarify do you mean 'cold water storage tank or F&E tank? As both serve as expansion tanks also, one from DHW cylinder and the other for your heating.

Shut off the flow to the cold water storage tank. Then run off the water in the cold water storage tank usually from the cold on the shower if fed from this supply failing that you may have to run off through the hot taps. Then wait and see if the Water level in the expansion tank stops rising. Hope this helps-Kevin.