Ok new place has some dubious mounting of radiators so I want to remove radiator, mount the hangers and then put the radiator back. Can I just shut of the radiator valves, remove rad and remount and re-presurise or am I going to have to drain down the whole system

You should be able to isolate the rad closing the valve and lockshield.

Undo one end of the rad and drain into a baking tray. Let off the bleed screw to allow air in the top.

Remount the rad and reconnect. Top up the system and bleed as normal.

Should be fine just shutting off the rad valves.