This morning I successfully drilled a 5 mm hole roughly co-axial through the pin into the cavity below it without breaking the drill, which at 106 mm deep drilling hand held was a distinct possibility !
Two objectives: A/ Introduce penetrating oil below the pin for it to soak in both ways, and B/ Possibly make a fitting to take a grease nipple into the M10 tapped hole to pressurise under the pin with a high pressure grease gun and maybe even force it out that way.
Well objective A/ did work but B/ is thwarted as the penetrating oil is leaking somewhere down there and there'd be no chance of getting significant pressure.
So back to heating and slide hammers. Another session with the existing slide hammer was fruitless so I made another hammer - actually repurposed the one I made to try and release the man hole cover over the bore hole. Had to weld on a smaller diameter 'stop' to match the 12 mm slider.
The 'new' hammer weighs 2.5 kg whereas the original was only 850 grams but still not effective. Obviously a longer slider lets you get up more speed thus greater inertia. Hunting for suitable rod a length of M12 studding came to hand and was pressed into service.
Still not moving the pin one iota

Amazingly the M10 thread into the pin hasn't stripped as it's taking an enormous whacking. The pin will be a decent steel no doubt and the nose is made from an HT bolt.
. . . tactical withdrawal to cogitate !