Sorry, I just realised the pasteboard links in my original message don’t play well with this forum’s interface. I have attached them again.
After much testing we discovered the issue was being caused by Cloudflare – https://www.stevejenkins.com/blog/2012/05/fixing-wordpress-admin-issues-with-cloudflare-page-rules/
We setup page rules to exclude wp-admin from various Cloudflare components and this issue resolved. Hopefully this post will save others some time.
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