Okay, the silence is deafening, either nobody knows or I've asked a really stupid question, I suspect it must be the latter.
Please be patient, being a 61 year old technology is not my strength.
Let me try to answer my own question and you can correct me where I'm wrong.
1, I get the customers email address.
2. I email them an invoice.
3. I hope they open the invoice online and press the pay now button.
4. They will (hopefully) fill out the details for GO Cardless
5, Eventually I'll get paid
and
6, from then on whenever I mark the as "done" Go Cardless will automatically debit their account.
Please tell me if I need to do anything else.
David