... the landlord wants to open up the wall on one of our WC to reach it in an easier way.

Should I give permission for them to this? Why cannot the landlord prefer open it up from the hallway? All they broke in the hallway was a few inches very close to the floor. Can I ask for compensation for the hassle of having to emptying it and having to host the plumber? How much if so? One or two months of rent!?