Heat and lube.
I used a heat gun at 500degC, got it nice and hot and whacked it with a mallet.
This was after letting it soak for a few hours in wd40.

My order is always:
Penetrating fluid, if that doesn't work add heat, if that doesn't work add shock in the form of a mallet.

Hasn't failed me yet, although my steering wheel almost got the better of me this morning.