Thank you Gal for your comments. It was not a cache problem. I have been using this plug in for more than three years and I had to make some changes in the early version to stop ‘Posts by category’ from appearing on a page only site. I was able to fix this by modifiying the code in the PHP file the issue was fixed in ver. 1.0.4.
At that time I had another problem that caused the ‘Page’ heading and bullets colour to appear white, which is the default font colour for TwentyEleven dark theme. I finally fixed this by using some ‘unorthodox’ code to the Sitemap page which allowed my font colour to be applied. That was a little over three years ago.
When I updated the plug-in from ver. 1.5.6. to 1.6, it appears that the WP Editor stripped out the ‘unorthodox’ code that I had written, so long ago. Because this happened late in the evening, I had hoped for a quick solution by reinstalling the old version. However, they do not appear to be available in the repository, SVN or elsewhere. When I looked at the Sitemap page in the Editor, I realised that the code had changed. I reinstalled my old code and the black background reappeared, but the Page heading and bullets colour appeared white again. Then by correcting the unorthodox aspect of the code, I was left with just the ‘Pages’ heading in white. The next day, to fix that I added code to the plug-in PHP file and now all is well.