We got a similar letter a few weeks back saying they owed us just less than £10 which was about the amount we were in credit when we left Sky many years ago. At the time we decided to leave it as it was a pretty traumatic experience trying to cancel.
Anyway, I rang the number and it was an automated system the only info they wanted was the ref no from the letter and within a few days a cheque arrived.
So I would say it's pretty genuine.
Edit
Bit more info
here.
Cheers
Grim