My understanding was that the limit was that the create command was per tag, but it seems like it's per registrar rather!
I have now fixed this on my system and hopefully it resolves it.
For other registrars I feel the wording could be clarified here https://registrars.nominet.uk/uk-namespace/registration-and-domain-management/acceptable-use-policy/ that it's per account not per tag. Many of the limits used to apply to tags and this is why I didn't pick this up.