How long since the start is this? Give it a minute or two, sometimes it takes a couple to start up especially on a weak VM. There is also a link on the desktop to see the log which would include any startup error you might be getting.
Thanks for the help Marten.
Very strangely - the exact same image ‘hangs’ on one of my Win 10 PCs and works on another of them. I can see (thanks for pointing me to the log!) - that it’s hanging during installation.
So - the .OVA worked well on 1 Ubuntu machine running VirtualBox, on 1 Win10 machine running VirtualBox and failed on 1 Win10 machine running VirtualBox - just in case this information helps anybody.
Thanks again, SB.