OK, it's a bit bizarre, but I have an idea.
When I'm at my desk (printer beside MacBook Pro), the wireless rarely works. When I'm away from my desk, for some reason it does seem to work.
I therefore deduce that the wireless receiver in the printer is weaker than in the Mac. The Mac beside the printer somehow monopolises or interferes with the signal (like when you have a poor FM reception on the radio, and the sound is clear when you're near the radio, but move to the wrong place and white noise breaks in).
So beside the printer I need to use the cable, but in the rest of the house I can use wireless.
To get the full range of printing options I have used the Presets option (see pic- can't show the detail, the menus disappear when I switch to Grab). The menu that by default comes up 'Copies & Pages' can be used to set the duplex settings (bottom of the list). I have one for 'Default' (single page feed from cassette), then others for: Rear Tray Feed; Cassette Landscape Duplex; Cassette Portrait Duplex; Rear Tray Landscape Duplex; Rear Tray Portrait Duplex. I set each up with the options menu, then go to the Preset menu and click 'Save current settings as preset', which allows you to name each preset. Now I can do all I want to, whether on USB cable or on wireless.
My router does need upgrading, but Canon need to put a high quality wireless receiver in, not some feeble thing that can't hack being in the presence of other devices. Best I can do. Not a solution, just a possible explanation.
