Most of the time this doesn't get called because source_required: false.
But sometimes it [does happen](https://app.bugsnag.com/yaycode/openfoodnetwork-uk/errors/66329690f4b6380007e8a4f8)
I have a feeling that source_required? could be moved to the superclass as payment_source_class.present?. But I don't know enough about this area of the system to try it...