I have just found the answer to my own question!
To ensure that the iP4850 prints from the cassette tray:-
In the printer driver which appears when you select Print, under
Properties | Quick setup tab | Check that "cassette" is specified in Paper Source
Properties | Main tab | Check that "cassette" is specified in Paper Source
Properties | Page setup tab | Print Options button | Check that the "Disable the paper source setting of the application source software" box is checked
Regardless of the fact that you have selected "cassette" as the paper source in the application software (e.g. Ms Word), you need to disable it
Either Word sends the wrong codes to the printer, or the printer scrambles the codes it receives. But whichever of the two it is, the printer then ignores the cassette and gets (or tries to get) its paper from the rear tray.
Disabling the application's ability to control the printer seems to fix the problem.
Hope this may help somebody else!