Thanks, Stefan. My users can now retrieve floating licenses. Something strange is happening, though.
As soon as the user selects a floating license they get a message 'Not enough storage space on disk'. This doesn't effect them - they can carry on and use Sparx (and Help\Register and Manage License key(s)' shows that they do, indeed, have a floating license).
However, as administrator I see the Keystore Manager app simply close down and, when I re-open it, the floating license isn't shown as allocated (although the user is still using one within their instance of Sparx).
This may be related to storage space on our network, of course - is it anything you're aware of, though, as a Sparx/keystore issue?