Finally, got it open. The pry tools are useless; eventually careful use of a small jeweller's screwdriver forced the two parts of the case apart enough to get the pry tool in and work it round the sides.
Two tips: the orange ribbon cable to the headphone jack was both stuck to the metal case, and held down by a piece of black plastic tape. This required care to remove, using a jeweller's tweezers. Second, it's not worth while unscrewing the circuit board to free the battery lead - just cut the wires and pull them through. The hardest part was re-attaching the ribbon cable connector for the jack. I used a paid of angled tweezers.