Ok this is what I just did. Filled system up, thermostat housing plug out and radator plug out. Filled rad til up to plug, filled thermostat housing until it was full. Ran it with thermostat housing plug out until water started coming out. Repeated process but released cap first before taking plug out. Took it for a spin and it was cold after 2 miles. Nearly home and - whats this? warm air coming out the vents? then it got hot and stayed hot for a little while then cold again, I didn't stop when I got home I carried on driving for a while and it seemed to be cycling between hot and cold. Got home, depressurised cap and took plug off thermostat housing and put more water in.
Should there always be water in the housing? It sounds like an air lock to me now but surely it cant be that hard to get a few bubbles out?