This will make you laugh Farmer C took his gate away so what did farmer D do but fetch another gate . Who is taking the pee !!!If I where farmer c I would call farmer b to remove his sheep. If he doesn't do it, call again. If he still doesn't do it, inform him you will, coincidentally, replace the gate shortly with a wire for easier access.
I have had something similar with a neighbour, his sheep come onto our land pretty much every year. I quite like the guy and I really don't mind, it's just that he always asks if he can let his sheep on our ground when they are already there. One time I told him to remove them as my gates are open. The wooly fückers didn't go on the road but they did find my silageclamp and round bales.