Aga hot water issue

The overflow from the header tank was leaking. The header tank supplies the hot water cylinder, which in turn is heated by an Aga flow and return. The header tank has one water feed in, one outlet to the bottom of the cylinder, an expansion pipe from the cylinder and the overflow. I replaced the float valve which is now working correctly. But the tank keeps filling. Only isolating the mains supply at source stops the waterflow. When the water to the tank is isolated at the tank, no water is added via the supply, but the water in the tank rises. Could it be back-feeding from the cylinder? Isolating the tank water supply and the cylinder water supply, gets the expansion pipe delivering water to the tank. I have checked the tank float valve and no water is entering. Really very odd - any suggestions?