Hi Peter
If that doesn't fix it, try releasing the .vdi disk image, delete your virtual machine and then create a new virtual machine using your old .vdi disk image.
That worked!
I notice you didn't include the quotation marks in the commands you quoted. That may well make a difference.
The relevant part of the manual is here:
and so did adding the quotes!
I can now connect to my Ubuntu desktop machine as if it was actually running Microsoft SQL server!
Now I just need to see if I can run VirtualBox as a server (I'll check the manual) and Management Studio under something like WINE.
Thanks Peter!