Hey Phil 
I’m a backend engineer on our credit decisioning team. For context there are a few things that could have changed your eligibility:
(a) we introduced in-house credit scoring (to replace using credit scores from our credit reference agency) in July - a lot of people will have been made ineligible (or eligible) by this
(b) we get new data from our credit reference agency every month that may change your eligibility
© some of our credit decisioning rules are based on internal data about how you use Monzo: this will change all the time and affect the decision you get
(d) your data may not have changed at all, but you may have been made ineligible because of changes to our rules over the last few months
I’m not sure which of these is likely in your case but if you send me your email, I should be able to take a look into it!