All OK thanks to excellent instructions. Now have Nitrokey working as expected running on Linux laptop.
Having imported required public key to another laptop the Nitrokey works fine with popup window asking for entry of PIN. Once entered I can open the encrypted device as on first computer.
Where I have a problem is when I plug the Nitrokey into an OMSC (Kodi) media device which is running headless. I have imported the public key to the OMSC device using SSH but for normal running the media device uses a remote controller. What must I do to get the PIN entry popup to appear?