Having sold my car last week, I cancelled the insurance. They told me to post the certificate back to them, but in all honesty I can't really be bothered.
This was sorted over the phone with them. I have to pay a cancellation fee of £48. I'd only had the policy 3 months. They say it'll come out of my account as a DD at the same time as my old one did, however they also tell me my old DD is cancelled.