What would they gain by doing this though - these empty threats? It's not like a debt that needs paying so they would get their money. The fence is already pd now.Let us know how it ends up.
I reckon, if it really is now permitted development, you'll receive a number of letters threatening court, however, no summons will materialise...
They could just say/admit that as the fence is pd, they won't take it further and save their time.