Thank you to all for the quick responses. Investigating the Ni500 further, it appears to be a particularly stupid router which implements NAT and can't be pinholed. OK for my purposes. This is the third modem that I've bought from this retailer (who is an honest and helpful bloke but in this instance will remain unnamed); all three have been subsequently abandoned by their manufacturers/distributors.
I've got printing more or less sorted. Gaining access to the printer on the other host was not merely a question of the firewall; I had to edit her /etc/cups/cupsd.conf security options to allow myself access. Trying to access the remote printer by adding it while looking at my localhost:631 doesn't work; I have to http to her port 631 (and then the printer is already there). (This makes sense, and is probably obvious to you network fiends but was a revelation to me.) =====Andrew
