Sorry to hear your predicament Nick.
I don’t know the answer or ever had one stuck but if I did I would be tempted to try freezer spray through a nozzle very locally onto the steel plug body to try and shrink it (as well as the WD40 thing). I might even try and rig up a tube around the plug to keep the liquid away from the ali head. I would try it first with the head at room temperature but if that didn’t work and depending on how desperate I was I might try it with the head progressively a little warmer.
Just like to reiterate I’ve no experience of this or heard of it being done. My concern would be creating a rapid localised temperature differential in the cast ali head and the fear of cracking. Might be worth getting the professionals comments before trying?
http://www.wickes.co.uk/Wickes-Pipe-Freezing-Spray-240g/p/424922#
