MMM?!?!
In reply to @christianp
The form has gone on to tell me that I've submitted my own personal details incorrectly.
In reply to @christianp
Ah, the unrecoverable error was to put a space in the middle of my postcode, like I did on the price comparison site, and because that's how postcodes work
In reply to @christianp
... and someone hasn't done their GDPR audit properly: opt-out checkboxes for SMS marketing