I am posting this in case anyone else has issues trying to fetch their sitemap using Google console. I have read several ‘solutions’ on these forums but they all seemed to be wide of the mark to me.
This is what I did to fix it. The problem was that I need to ref-verify my website by adding a new meta tag in the header, once I’d done that I was able to fetch my newly created sitemap.xml
I hope that helps anyone else who experiences this issue which had me puzzled.