I mostly get sober,reasonable customers during the day.The only reason I use the referral codes is because I had passengers book me through the app as a cash job but then on the way wanted to pay by card.Had I processed the job on my own reader I'd be paying 13.75% commission(12% plus 1.75% to Sumup).So it's easier just to get them to register the card on the app.They get a tenner and you get a tenner.
I'd say others might have better numbers but can't be arsed.