have you consider to integrate with https://toshl.com/ ? It’s a budgeting app that lets you connect all your bank account (for example, nationwide, BA american express, capital one, etc…) so you can track all your expenses across accounts. They’ve just lunched the integration with UK bank accounts, I wonder if Monzo will be available on there as well.
You’d need to ask Toshl if they can integrate to Monzo not the other way round. Monzo have an open API Toshl can hook in to but it’s up to them if they have the appetite to do so