Thank you Calvin and Gerhard for the suggestions. I had thought that I had set up a port forward in the router (which is of course NATed) so I could ssh into the computer, but something appears to have reset that, or maybe a new install changed the firewall. These things are rather hard to test from inside, unless one has a remote shell account to try them from. But vnc is not an option when something has broken. Teamviewer could be interesting, but the linux version needs wine, and that is something we have no experience of either...
cheers
ant