Welcome to the Q&A Forum

Thanks Donna,  I've already tried switching back to the default theme and no difference.

However your link does show that Wordpress itself adds the canonical link to every post and page by default so that'll be where it's coming from, rather than the theme.

Thanks.

Myself and a friend (who is a competitor) both had the same problem.  Doing a URL Inspection in Google Search Console - specifically the Test Live URL thing - fixed it immediately.