Airlock in pump?

We just had an immersion heater leakage fail on an A-class. Drained the whole system, replaced the immersion heaters, refilled and job's a good 'un... got hot water

But whenever the pump runs now, it has a loud water splashing/gurgling sound coming from it which it never had before, sounds to me like there's trapped air in there.

The standard procedure from what I gather is to untighten the bleed screw on the Grundfos pump by about half a turn... let the air out until it's just water coming out then re-tighten...

Only problem is, no matter what tools and how much force (including hammers) and WD40 I use, the bleed screw will not budge at all and it's starting to damage the metal slot on the screw. How do I get the bloody thing off?! Is there any other way to get the air out?

P.S. Yes - the tank is completely full and the header tank filled to the ridge, there's no lack of water in there, just trapped air I think

Well if you can not undo the screw.
Try cooling the unit. Then the pump will cool down and you may undo it.
There is an O ring which is rubber.
This may be stopping you.
Other that that you will have to loosen the pump valve / connection above the pump.
Again be careful if hot.
Another way is to.
Switch the unit off, wait then on. The pump will run for a time.
Do this a few times it may clear.
Or turn pump valves off.
Place a towel under the pump.
Remove pump or pump head.
May get screw out better that way.
And you could try opening pump valve up very carefully to vent the top pipe.
Mike