Hi Nigel,
Got it.
After copying as you suggested still didn't work until I went to Edit Plant and changed the Content Template to "Template for single plants" - these senior developers are a bit slow!
Now I've been struggling with another problem -the sticky nav menu that I'm using (from GeneratePress) disappears half way down the page.
I've been in touch with GeneratePress and after some checking they say that the problem may be with Toolset. Do you think you could take a look and let me know if you have a solution.
I looked for a page where I could see the issue with the sticky nav and saw it on the plant names page (hidden link).
With my browser dev tools open I could see the issue, namely that the nav element has position:fixed, but at some point when scrolling the page it switches to position: absolute (which appears to be triggered by a file sticky.js).
I edited the page, and I couldn't see any content related to Toolset.
So I disabled all of the Toolset plugins and reloaded the page—where I observed exactly the same behaviour.
Meaning that this isn't caused by Toolset. I don't know enough about this Generate Press sticky nav, but I think you'll need to go back to them.
This video may help (it was recorded with Toolset de-activated): hidden link
In it you can see how inline styles for position are being updated via JS depending on the scroll position.
I sent Nigel's response to GeneratePress and I think on the basis of that they finally managed to sort it out.
But Nigel did the work - and didn't just pass the buck as is usually done.