Thanks very much for the tip, I changed the background and that has sorted out the problem. Presumably it shouldn't happen with a transparent setting either but I can live with having a coloured background.
I have a standard starter CSS file I always use when starting to build a site which has all the basic CSS things setup in it. One of the things in that file was an explicit instruction to set the background to transparent. Having read this post I deleted that from the style sheet and haven't had the problem since, I didn't need to set a background colour, just to not set the background to transparent.
Thanks again for your help.