Next, I enabled a static route in my wireless router, so that requests to the printer's IP address went via the 'gateway' IP address of the iMac -- this address was the IP address of the AirPort adapter. The iMac seems quite happy to forward IP requests from the AirPort adapter to the Ethernet adaptor. I can now treat the printer as a networked printer (despite it being on a different subnet), and hence set up the printer on my Windows laptop. Hope that kind of makes sense -- I am happy to expand on any detail if anyone's interested!

