Quoting Pendennis: Without engine running, remove nose cone, unbolt and raise header tank, fill it halfway, you need to vent for a while maybe an hour, initially remove top rad plug, but when water comes out the rad is vented. Leave header tank cap off whilst tank is highest point of coolant circuit, occasionally squeeze top hose to rad, should be job done. Then reinstate tank but ultimately you should be looking at relocating tank to highest point of scuttle bulkhead.That trick with the expansion tank is clever. If you have to do this a lot you might be able to use something like a bike quick-release skewer to hold the tank in place...
Jonathan