I relogged twice but there was no improvement, I restarted my machine, cleared cache and still no improvement. Eventually I gave in and waited 3 or 4 hours before trying again, at which point the problem seemed to have gone.
Would love to know if anyone else is experiencing this and why it may be happening? (Obviously, I realise that the answer to that may well be 'Because this is Second Life run by Linden Labs, what do you expect?')
The other problems I'm seeing friends come up against is being unable to open inventory and inability to create notecards and other actions associated with Inv.