Finally got it out!

loads of WD40, brake cleaner, patience and many, many hits of the persuader finally broke the little b*st*rd.

there's a crust of rust on the section of the pin that resides inside the brake caliper. And there was a puff of rust coming out when it finally came loose.

So I'll be thoroughly cleaning the pin with some sanding paper and greasing it up copiously. The caliper itself will get a thorough clean as well. Just to avoid similar problems in the future.