More troubleshooting: I've now noticed that several of the HTML files on my website, including those uploaded several years ago, are now affected by this incorrect display problem, having been OK previously. When I open these files on my PC HDD, rather than from the website, the displays are OK. So it looks like the problem is with the website host, which is names.co.uk. I'll give them a call next week. Thanks for your inputs chaps.