I hope you are well.
My suggestion is to improve the security of the Monzo service via the integration of a Random Number Generator (technically, you might be able to use a sudo-random number generator) in the App to generate truly random PINs.
Although I like the idea that you are able to change your PIN at an ATM, which I did, I am a bit sceptical with regards to the randomness of PINs we users chose and whether there might be a systemic security flaw in this approach because card owners do not use the full spectrum of available PIN combinations (see below examples). For this reason, you might want to add a simple Random Number Generator that generates PINs for users to the app. This would allow everybody to create a truly random PIN for their cards and could help increasing the security of the Monzo Banking solution.
I am pretty sure a lot of “nerdier” cardholders selected “1337” as their PIN;
Thinking along similar lines, I would suspect that many of us used meaningful dates, e.g., birth dates, as their PIN. This could mean that (i) the number combinations “01”-“31” (for days in a month) as the first two digits, and (ii) the number combinations “01”- “12” (JAN-DEC) as the third and fourth digit, have statistically a higher probability of being used in a given Monzo PIN.
Probably you will also find some cards with a PIN set to “1234” or “1111” etc. (If this allowed by the system)
Users might use their Smart-Phone-Screenlock PIN as a PIN for the Monzo Card; i.e., hacking the smartphone lock screen key could hand a hacker your Card PIN.
That’s it; appreaciate our feedback and comments.