Thanks for your reply esmi.
Yes, for sure I take your points re plugins and rss. Isn’t it still strange though that outwardly the site works fine, and that it is only the /wp-admin that is returning the 500 error. I would have expected the rss issue to be on published content and to bring down the published site.
I’m looking at having to do exactly as you suggest and re-chassis everything onto a fresh install, and would rather avoid it if possible.
Any other thoughts greatly welcomed.