I've just checked to see if there are any answers to the above issue and then found I hadn't posted it.

However, I decided to post it anyway in case someone else accidentally comes up against this issue.

Here's what it seems was the problem.

I had the layers and guides window open and I noticed that the text I just posted was in the guides, not the layers. So all I did was grab the icon from the guides and drag it down to the layers for the same page. Problem sorted.