Hello,
I recently returned my kit after leaving BT, the router and the TV box, however I am still receiving emails saying that the TV box has not been received even though they were returned in the same package. I have the proof of postage still for this.
On the proof of postage there should be a Royal Mail tracking number, go to the Royal mail webpage and print proof of delivery and then ring 0800 800 150
I have had the same issue.
I sent BT a DM on twitter and they were quickly able to check and update their records.