I know about the scenario simulation, and when I have corresponded with the support team it is where they have pointed me. This in spite of saying I have read it and explained why it isn't doing what we're after.

In its current form, it is not adequate. We have to manually set up the resource to run it on. Once run, there is no easy reset, to re-run it (if, for example, the response was handled incorrectly).

It can be argued that mocking all behaviours should be carried out our side and not involve the GoCardless sandbox. I am open to that, but note we have not seen any existing libraries handling this.

And the behaviour of other payment processors suggests this is not the most common approach to take.

Edit: As samples of all events/payloads are not present, we have to record production data first in order to get the data we can use in our mocking and testing.