thanks Jdm. I was thinking calipers and yes will probably pick up a press as suggested.
will try just to break off the rest of the glass then without mangling the case. seems like not worth the hassle to try to work out the pushers as they seem to be fairly fiddly to deal with so best avoided for me.
You don't need any special tool to remove pieces, tweezers will do. Measure size with digital calipers. But you need a press to put it back, you can get a cheap one on Ebay, just don't expect too much from them. Same for the crystal, there are specialized shops but Ebay is good for that also.