It really sounds like a browser cache issue. I can't think of anything else that would explain it. Try visiting the site with your browser in private or incognito mode.
You can also try adding a dummy parameter at the end of the URL to convince the browser not to load the page from its cache.
I assume that you've tried manually deleting all files in the MODX core/cache directory.
Thank you for your input. I have tried all of those suggestions to no avail. I have also had a look at the chunks and templates in the database itself in case that wasn't getting updated correctly, but everything looks good there. I've also run your SiteCheck extra which showed no errors and checked both MODX and the server's error logs which showed nothing either.
I have also found that if I delete the content of the template or change to a different template the page doesn't change in the browser, using multiple browsers on multiple devices.
Could this be caused if there was something wrong with the .htaccess file? I have Friendly URLs "on".
Thanks - Mark.
[ed. note: zaphodx last edited this post 3 months, 4 weeks ago.]
I turned Friendly URLs off and the problem has gone, so I'm guessing the .htaccess file may be the problem, but it looks OK to me.
It was the .htaccess file. The old one from the previous site was still there, doh!