Well, I found it. Gocardless have designed the most convoluted way to show the mandate ID in their new dashboard.
- Go to Customers
- Select your client
- Click on the link to the client’s bank account
- At the bottom you have “Mandates” and a reference number. Click on the ref number
- On top of the screen the Mandate ID is shown above the DirectDebit logo.
After I entered that ID number all seem to be ok. I managed to collect an invoice.