no i haven't done a compression test yet. i did think of that, but cant find the tester.
i tried yet again having it running, just with a brand new cap.
notice this time was, the top hose was bloody hot, bottom hose ice cold!
bottom of radiator ice cold, top of radiator hot!
going to pull out the thermostat and check it manualy in some water as Danny M said, and if thats ok, i may have bought a dodgy radiator [:u]