Thank you for your answer, but I'm afraid that it didn't work.
I right-clicked the MG6400 series icon which gave a menu, on which the Properties item listed the main headings of what the printer will do.
The Printer Properties item yielded a 'Features' list that included Double-Sided ... Yes and Paper available alongside a space for a list to fill in entries. Only A4 was listed. But I found no way of adding another entry.
. For everyday printing, A5 is available as a paper size.
The error message I have been getting when trying to start an A5 duplex print is
"A paper size that is not supported by the automatic duplex printing function is selected.
Stop printing, uncheck Automatic on the Page Setup tab of the Printer Driver, then print again."
The wording of this error message implies clearly that it will do duplex on other paper sizes if only you know how to ask it to do so. My previous printer was an MG6150 and it did this without this fuss.
Russell, your help is really appreciated.