Thanks JimP and Moonshine.

I am using Windows 10 Pro, Office 2010 and an Epson XP-630 printer.

In Word I have scrolled all the way down the list of paper sizes and only A3, A4 and A6 are available - not A5

I was unaware that the page sizes are controlled by the printer driver. The Control Panel and other third party and Epson software tell me that the printer driver is up to date.

The paper tray holder of the XP-630 printer has markings on it for A5 paper so I will have to check further into the printer drivere=. I will remove the driver and then reload it to see it things improve.

Once again many thanks for pointing mr in the right direction.