Sorry to revive an old thread but I found this interesting - was in the Apple Store today upgrading mine and my partners phones on the programme. My partner hasn’t gone entirely full Monzo yet but was going to change her direct debit at this point to Monzo (she can’t do a CASS yet for multiple reasons)
The person at the Apple store recognised the sort code (!!!) and advised against using Monzo as they had “had so many problems with people getting rejected”
Wonder if this is anything to do with Monzo only being visible on transunion? (She told me Apple/Barclays check Experian)
Having said that, I used my Monzo account (as I am full Monzo so had no choice) for my application and it went through fine… But then, I have been on the upgrade programme for 3 years so my checks may be less stringent than a new applicant?