Thanks. I'm almost there, but there's a problem. Although I am using the A3 sheet, when I choose view background, SE will only let me adit the A4 sheet. Any idea why?
What background sheets have you got?
Sounds like you only have an A4.
You can create several background sheets - one for each size of drawing.
Then on the working sheet RMB on the sheet tab and select Properties. here you can set the sheet size and background to use for that sheet.