Network printing after moving from 17 to 18

I'd been printing (via wireless router) from my F17 laptop to my upstairs F17 desktop with USB printer for over a year.

Once I moved to F18 (using fedup) a week ago (both machines), I now get "CUPS server error - server-error-service-unavailable" whenever I try to print. Same problem when I boot either or both machines back into F17.

Could someone please talk me through diagnosis and resolution?

Thanks.
Tom

---------- Post added at 07:57 PM ---------- Previous post was at 07:17 PM ----------

Note: same issue when using Gnome in standard, Gnome in fallback graphics, and Cinnamon.