I think I’ve found another reason to not use my Monzo account.
I went to the help tab and tried to find a way of contacting Monzo by searching for failed transactions. No luck with that, unfortunately. I then remembered a way to get to chat was to search for coronavirus, which brings you to a help article with a ‘get in touch’ link. It’s not exactly intuitive, but it works.
I then stated that I was unable to make an online transaction for the reasons in my previous posts here.
First response from the COp was to ask if it was contactless or Chip and PIN 
It was escalated to a payments specialist who jumped to the wrong conclusion, perhaps because he wasn’t in possession of the full facts.
I explained the full facts and said I wouldn’t bother using Monzo online any more, and the next guy wished me a good ‘rest of my day.’
I’ve been with Monzo since the prepaid days, and have had reason to contact them more than I have with any other bank I’ve been with, and I can’t remember ever being terribly satisfied with the responses I’ve received, and this latest interaction wasn’t great, either.
Anyway, I’ll stick around on the forum, because It’s by far the best thing about Monzo, it’s just a shame it leaves so much to be desired as a bank.