Thank you all for your replies, My system is back up and running and so far the noise of air in the system has greatly reduced. There is still a bit of air in the system, but I think this will clear as I bleed the radiators( & pump) over the next few days.
I am not sure why the pump started to behave itself eventually. I tried several things, just turn hoter water on, just turn heating on, cut power to the controls and reset the timers, turned the boiler off/on, even tapped the 3 way valve & bled the pump while it was running. I suspect 30kw is right in that there was an airlock, as quite a bit of air come out from the pump.
Note, that I did not touch the 3-way valve when draining, not sure if you are supposed to.