No need! We'll liaise directly with your old supplier to let them know about your switch.
In a few weeks' time, we'll ask you for meter readings. We'll use these to agree on an exact point at which we'll start billing you for your energy, and your old supplier will stop (so you're never paying for the same energy twice).
Your old supplier will then send out your final bill to close your account (or let you have any credit back) — sometimes this process can take up to 6 weeks.
The one thing we can't do is cancel your old Direct Debit — that's between your bank and your old supplier — but we do recommend you organise this as soon as you've settled up your final bill with them to make sure no further payments are taken.