Hi Graeme,
Sounds like an issue with your text editor. If you have site admin permissions, would you be so kind as to share with us:
- The version of Moodle you're using (e.g. 3.4, 3.5, 3.6, and sub version)
- The name of the text editor you're using (TinyMCE or ATTO)
- A screenshot of the issue could also be useful
You may want to check Site administration > Plugins > Text editors > Manage ... to see which one(s) is/are enabled, and maybe switch between these, to see if the problem persists in another text editor.
Also, this sounds like an issue to be moved to the Other components > Text editors forum.