Hi all, I solved this issue.
Not sure if it is relevant as it depends on whether you are Admin of multiple Facebook pages on your personal Facebook page. However, I found that if I was on my business Facebook page on Facebook then that particular page couldn’t be selected within the dropdown menu of the plug in. It was very strange so I switched to another business page connected to my personal account and the one I wanted to appear in the menu actually appeared and so I selected it. It then wouldn’t let me publish (or whatever the confirmation button was – I can’t remember now) unless I was viewing my own Facebook page. So I switched to my personal Facebook page and hit ‘publish’ (or whatever the confirmation button is called) and it worked.
Sorry if I have explained that poorly but it’s the only way I know how to explain the unexplained!!