Thanks Victor,
Actually I should have been more explicit - we were unable to see the MPSK value when exporting (and even when viewing device details within Guest), there was a setting in Guest Configuration > Guest Manager called "Password Display", I needed to tick this. I don't recall having this problem before and we have just upgraded to 6.8.5 so I wonder if this is a new thing. Anyway, it's ok now, apart from we another issue with the SMS and email (non!)auto-send, but I'll create a new topic for that....
Guy