I rang the number again on the back of my card and got through to a super helpful agent. Having confirmed the verification e-mails were being sent but not received we tried several different ideas and in the end nothing worked so I asked to change my registered address. This was done by sending a picture of my photo ID to email@example.com. Within a few minutes the same agent rang me back and my registered e-mail address had been updated. When I tried to verify again using the new e-mail it went straight through 1st time.
My thinking is that this is not a problem with Monzo per se, rather the Monzo system simply doesn’t like my original e-mail address but nothing to do with spam. It is a weird one.
Once I talked to the Monzo agent things were solved very quickly indeed.
Thanks for all your help and tips here in the community.