Well that's been quite a tough few days coding for this very enthusiastic but amateur developer. Anyway, managed to get "webmentions" working on my @eleventy based blog.
Thanks to a whole heap of people but especially @sia and this article
https://sia.codes/posts/webmentions-eleventy-in-depth/
and also @mxbck and this article
https://mxb.dev/blog/using-webmentions-on-static-sites/