so what is the operating head on your system?
You must have missed my reply to you a couple of posts above... plenty of head: two-storey house, boiler on ground F&E in loft - now on platform 1.5m high after one early attempt to fix pump over. It helped a little but didn't stop it under all circumstances.
tryitandsee - I think the pump can definitely be ruled out as there's no more air in system after fitting 28mm vent. I simply came back to this topic to report what fixed my problem in case it might be of any interest to anyone else.
I wonder how many systems get put in with 28mm to and from the boiler with 22mm vent? I've only been familiar with four other CH systems but I'm pretty sure they were all 22mm for both boiler and vent in smaller houses.