I have got a solid fuel rayburn which has been plumbed in, according to
the instructions, with a gravity fed system providing hot water to a
cylinder and with a towel rail as a Heat Leak as per
http://www.aga-web.co.uk/raytech/dhw3.htm

The problem that I have is that the cold water feed is ultimately
getting hot, and is heating the water in the header/expansion tank. I
would have expected the system to use the vent from the flow should it
need to expand, and not use the cold water feed from the expansion
tank.

Has anyone else had any issues with their rayburn plumbing, that can
offer any advice?

Thanks
F_Puuuu