Hi all, thanks for helping me out here.
Relish, in the end were able to help me. I had a good chat with them on Twitter first about the problem, and after calling them up to clarify, it turned out that we could infact do what I wanted to over the phone. For some reason, the website doesn’t accept the Monzo sort code, but their computer systems on the phone is.
So, I’m now all set up - and I can see on the Monzo app that the active bank check has gone through, so it’s all good!
My recommendation to anyone who’s in the same bost - just call them up, and they can sort it out for you.