Briefly this is the mystery:
If I connect my laptop to my home network using an ethernet cable it can 'see' the printer but if I connect the laptop using wireless it cannot find the printer (neither can I ping it).
Setup details:
Computer A has two ethernet ports. Computer A connects to the router via an ethernet cable (eth0). Computer A also connects to the printer (eth1) - ethernet port and printer are on a subnet.
Computer B also connects to the router. At first it could not find the printer until I set up a static route on the router which cured that problem.
If the laptop is connected to the router via cable (eth0) it can find the printer but when connected via wireless to my wireless access point (which is connected to the same router) it cannot see the printer unless I add an appropriate line to the routing table on the laptop.
Can anyone explain, to satisfy my curiosity, why this should happen?