Water appears to be flowing past the top of the radiator when you run it with the cap off, thermostat is opening. It was fine before I drained the system to change the grommet.
I thought it might still be an air lock but am no expert.......
The pipe coming out of the heater was stone cold so I opened the system at the highest point (the pipe going into the matrix) refilled the system and then I got hot water coming out of the matrix but only as far as the bottom hose.
The fact it was running perfectly before all this makes me think it could be an air lock.