I finally managed to get it off this evening - eventually resorted to using a wide flat-bladed screwdriver, gently working it down between the bead and the rim then twisting to prise them apart. Once a small section was off the shoulder, the rest came away easily. And once the bead was in the rim well, it was easy to get it off with just my thumbs - although I only did one side, just enough to get the tube out.
Then discovered the rim was already taped with tubeless tape, and since I have some spare tubeless valves in the parts box, and a large bottle of sealant, I decided not to bother putting the tube back in.
Tbh, the thick layer of tape might have been part of the reason it was so hard to get off.
Considering doing the other tyre tomorrow.