Anybody got the technique?

Cheers

Tapped mine out with a screwdriver and a hammer, with the shell balanced on a workmate. There should be just enough of a gap between the two halves to do this; I think they were still useable afterwards too, although they are quite soft and easily damaged.

ah not enough gap for a screwdriver

There’s no gap between the two halves on mine.
I managed to force a small, sharp screwdriver in like a chisel.
Once I’d created a gap, I worked it round.
I keep meaning to file two notches in one of them to make it easier next time.