There is a disconnection department but i cant remember where i sent mine to. I did phine the number on my bill, 08700 700191. You can call 191 from your moby as its free.
I would suggest you politely expalin what has happened and get the address to cancel.
Also, I would send letters on different days so your post doesnt get mixed up.
BTW: i cancelled my phone the other month to get a new number, They contacted me with a much better deal than what i was on at the time. A free Sharp GX30 was delivered to my door aswell. Saving £100 from instore purchase.
T-Mobile, O2, Orange, 3, - no thanks