How to remove concrete paving stone from mold?
Thanks to all who provided suggestions in comments/answers - I've actually succeeded in getting it out before having chances to try them. They'll definitely come in handy in the future I think though - and the muriatic mention from Shimon will definitely be useful as I wasn't aware of it and the ability to etch concrete could be very handy! Plus it would have allowed me to save a casting I lost earlier where excess concrete in just the wrong place caused it to be nigh-on impossible to de-mold without destroying it.

Ultimately I used a very low-tech solution of inverting the mold on the floor and standing on it, shifting my weight to flex the "bottom" of the mold  and this caused the slab to drop enough (the slab wasn't flush with the top of the mold so there was a few cm of room for it to drop) that I could get a butter knife in the edge and pry the slab loose.