I replaced the rad a few days ago and bled the system. It runs fine after bleeding but once I turn it off all the water drains out the thermostat housing. If I run it after that it overheats, unless I take the bleed plug off the housing and bleed it before every run. Any guesses as to the issue? Cheers

Have you taken the stat out to see if it's ok.
It's amazing how much coolant you can keep on trickling in via the thermostat housing until it eventually gets full. Presuming you are doing the final top up via the stat housing?