Grahams should be able to help you (Beckhampton st)
You don't undo the painted hex nut, the rad tail will have a hex or splines inside. Often needs to be heated to reduce the effort to undo it.
To be clear, can you measure the diameter of the inside of the tail - ie the hole, visible in the pic you posted?
Most rad valves are "half inch" in there, 3/4 is a (expensive) possibility.
You don't undo the painted hex nut, the rad tail will have a hex or splines inside. Often needs to be heated to reduce the effort to undo it.
To be clear, can you measure the diameter of the inside of the tail - ie the hole, visible in the pic you posted?
Most rad valves are "half inch" in there, 3/4 is a (expensive) possibility.