I suspect gripfill may have been used dilalio! We're now using a guitar string because the cutting wire was too thick and improvising with pencils as handles, until some arrive on Saturday! To be fair, I did manage to get a little clockwise and counter-clockwise movement this morning. I had wondered if using our heat gun might help.
Keep manipulating and cutting it should come away eventually. Wouldn't use a heat gun but you can perhaps put a plug/bung in the basin waste and fill it with boiling water. Might soften up the adhesive enough.
You could also tape up the worktop for protection and use some plastic tiling wedges, tapped in, bit by bit, around one half of the basin, as you cut. Guitar string around some dowel with electrical chock block to secure the loop of wire back on itself would enable you to get a good grip and sawing action.