Consent or not, an order is an order.
The parties to the Fencing Order are (presumably) you, and the outgoing landholder/ seller.
The incoming buyer is (presumably) not a party to the Fencing Order.
Your information is (presumably) coming from the incoming landholder/ buyer.
(as in "Oh, no, I don't know anything about that, and I'm not paying...")
That person could be a victim of a fraud on them by the seller.
My suggestion - look into the prospects of enforcement of the order against the outgoing landholder.
That's not likely to be a DIY though.
It might also cost more than the dollar value in dispute.
Everything else is more likely to be dispute between the buyer and seller,
and/or the buyer and whoever did the transfer (a conveyancer?)