Best answer by woz
There's a post HERE that should help.
You need to contact them through HELP in the app, ask wattbot and type "message the team" to give them all the info, new address, new owners contact details date etc. You'll need meter readings when you leave and it won't hurt to take a photo of the meters too. (and I think the new owners will need to download the PP app and register so they can either switch away or stay with PP. (I think that's how it works, but as ever I'm open to be corrected)
It's best to leave the DD in place or they won't know where to pay the refund, but that's up to you. The process can take a while, anything up to 6 weeks.
(If the amount they owe you is substantial you could always ask for a partial refund and I suppose it also depends on whether you'll be moving to PP in your new place, but I don't know how that works?)