Sure, most people have active direct debits set up, but isn't the number of those are set up solely through the internet somewhat smaller? Furthermore, most people seem to associate direct debits with a certain class of business, usually utility companies. In my experience, it just isn't common for someone to set up and manage a direct debit online.

So yes, whilst one-off payments are significantly larger, I'd still be wary of a reduced conversion rate through GoCardless. Of course, if I had a very narrow margin, your incredible rates might easily redeem the difference in revenue, but without hard numbers it's difficult to make a business decision.