Moving the site from Google to WordPress was accompanied by a change from HTTP to HTTPS. The Ordinance Survey mapping application used here validates the account using a URL and key code combination. The new URL was being rejected by the app which is now out of support.
In the middle of Covid lockdown OS Customer Services have an email backlog of more than 2 months, so this was not the best way to get help.
Somehow the message reached the API team and then this was fixed the same day!
Most of the UK backpacking posts here use OS maps to show the way, so things are back to normal.