Just to update this, I think I've been done.
From the conversation I had yesterday on the phone, it started out well. The guy sounded positive when I made the request, and it seemed like they'd be able to pay the monies direct into my account, rather than send a cheque out.
As I was chatting, he asked for a new meter reading (I was expecting this, so was prepared). Then popped me on hold for a few mins.
Then his tone changed a little, he sounded a little worried (I work in a call centre environment, it's easy to spot someone who expects to be shouted at).
It turns out, when the retar... erm guy came to read the meter in November, he took the details down wrong (economy 7 plan, low & normal readings upside down). So they had to adjust that whilst I was on hold, and after updating everything, I owe them £40.
But they're keeping the DD the same, as I'll use less over summer anyway, and I've changed to 3 monthly bills (the bill with the wedge in credit was up until last November, but we only got it the other day).
Oh well, it's money I never had, so I'll never miss, right?
