Freeing an Airsporter trigger block

[robs5230]

Against my better judgement I've been talked into looking at an old airsporter, which is not cocking. Lever only moves a fraction then stays solid.
Guessing the piston head has seized in the cylinder.

Last time I worked on an airsporter was over 30 years ago and they've been a bit of a distant memory.

After removing the cocking linkage, the trigger block is absolutely seized

So far I've used penetrating oil and a mallet on a bar through the hole in the block, and nothing is moving at all.
I'm reluctant to use heat due to the painted finish on the trigger block.

Guessing there's the issue with corrosion of sorts, between dis similar metals with the block being alloy and the cylinder steel.

Anyone have any ideas / suggestions?

Thanks in advance. Rob