You say "It’s not the b t box".
I don't share your certainty, but if you say so then it must be the TV or the cable.
You say that "the cables carry both sound and vision I deduce that it’s not them".
I don't share your certainty, but if you say so then it must be the TV.
If it's the TV, then it's either faulty, or it's not compatible with the BT box. Since this isn't a new TV, I wouldn't have any expectations of it working, and you have no recourse to a retailer or, unless it's under warranty, to the manufacturer.
If it were my TV I'd take it back to the seller and ask them to demonstrate it with a SCART source and an HDMI source. If it works with a SCART source then you have a gigantic mystery. If it works with an HMDI source then you have an incompatibility. Either way, I'd abandon it and buy a new TV with a nice new warranty.