Thanks Steve. That makes sense. Yep, the threads in those additional holes have completely gone.
I've been using a dead blow hammer which is rubber coated. I can't hit it from behind because the sump is in the way. So, I've been hitting it from the side, rotating the pulley and then hitting it again etc. to give it same force from all sides. It's not worked so far.
To give you an idea of how badly it's jammed in there... the space around the spacer on which the crankshaft pulley's main bolt sits, was completely filled and looked like a solid lump of metal! I had to use a tiny screw driver and hammer to remove the rust in order to reveal there was some space between the spacer and the pulley itself.