My order is:

Content
System Cache
System Sef

How can I modify the ordering of plugins to see if this fixes the issue?

Thanks!

OK, I realize that was a stupid question now and I can just change the ordering inside the plugin itself, which I did. I put cache last, turned back on SEF and URL rewrite, cleared cache and tried again....but unfortunately this didn't resolve the problem. Same thing happens.

Sometimes they appear, sometimes they disappear. Seems to have something to do with being logged in or logged out too.

I don't know. Maybe it just has something to do with cookies since it doesn't seem as though the issue is occurring in incognito mode.

So basically cache is not clearing in your site.
Is the caching plugin enabled? Also if you hard refresh the browser is the issue resolved?

Yes, cache is enabled (Cache Time - 15 ON Conservative Caching) .

And yes, it does seem the issue is resolved after hard reset of browser.

Although I'm just happy if it's resolved, does that mean it's system caching related or browser?

Many thanks!

Check the plugin (on the plugins list) - System - Page Cache and make sure that Browser Cache is not enabled.

Use browser cache was indeed enabled. Hopefully that does it. You're a wizard.

Thanks very much!

You 're welcome Scott :)