When I asked them originally, all they told me was that the box would no longer work, which I wasn't convinced was true and is the reason I posted here.
@kwaker5 that incorect response suggests that they haven't looked into your actual contract and account.
Customer Service "guides" are quite ignorant of facts. Even if they determine that you had the box prior to the date mentioned they'll quite possibly insist that the box must be returned but it's yours to keep.
If you've got the Pro box then yes you'll have to return it or be charged as that wasn't available until after December 2019.
I took a look at the box and I think I have a BT TV Recordable Box G4. Any idea if they will want this back or whether it will work if I leave?
@kwaker5 BT normally apply the date based criteria in determining whether a non return fee would be charged, BT also would normally (as part their recycling policy) ask for return. Of course if you are the legal owner of the box it is optional to return.
Two simple ways of determine which box you have on working boxes is to open box HELP app and press I (nfo) button on remite and you get a About your Youview Box display or use the Box home page Settings >Information & Reset >Device Information
Just keep the box and if works it works! If you get a bill for it you then send it back and BT will refund you.
Everything else has been repeatedly said so it's now up to you what you decide to do.