The older boxes may store your recordings etc, and still allow access to particular channels, so until they conk out or you can't access something, Talktalk would expect you to carry on using what you have, @Tonyjpayne.
If you can't access eg ITV X, you will need a replacement box, but check the specs because it is likely to be the 4K.
You will pay £5 a month rental unless you get it discounted (instead of the previous TV charge) and may need to start a new contract.
So you need to take action and call 03451 720088 - open till 6pm today, not Sunday.
Gliwmaeden2, a fellow customer.
