This whole thing is totally deranged.

I *do* have a One Login account with this email address (entertainingly I personally have *TWO* "One Login" accounts)... but read that carefully: "[...] for HMRC".

... and keep reading: "If you use GOV.UK One Login elsewhere [...] you cannot use it for HMRC yet."

Which means this is either strange for just-me, or it's a global "not for HMRC yet". If it's the latter, why did they even offer it as a login method?!

Screenshot of a gov.uk site.

Heading: You do not have a GOV.UK One Login for HMRC

Text:
There is no GOV.UK One Login for HMRC with attie.grande@argentum-systems.co.uk.

If you use GOV.UK One Login elsewhere, like Companies House or the DVLA, you cannot use it for HMRC yet.
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