It seems clear from Lady P’s excellent response above that you have a stalemate here. The new owners can (probably) remove the fence without anyone’s permission, but she can’t put up a new one without yours.
I would say this situation would benefit greatly from a mediation with a view to reaching a compromise that allows her to reclaim her area of exclusive use without impacting unfairly on you.
Compromise and negotiation is the only way you and she will achieve an outcome that doesn’t lead there being no fence at all. But if that would suit you, then so be it. She probably can’t ‘move’ the fence without your approval and the sooner she accepts that, the better.