Ah, one of their more expensive deals :)
I was with Scottish Power until April for around 2 years and in all of that time their web site allowed me adjust my DD payment. It had a automated over payment refund based on a payment threshold I set (from memory, it could be as low as £10).
I provided a monthly meter reading and within a couple of minutes a new bill was generated showing if I was in credit/debit.
I moved from Scottish Power in April when my fixed price contract ended and they were no longer competitive with their new/current deals. Based on my previous annual consumption of gas/electricity I'm on 12 month deal with another supplier that is approx £150 cheaper than Scottish Power can offer.
Which figures were you using? You cannot use the comparative figures for the mythical average household that they provide on their web sites (unless your consumption of energy matches this average household).