As rb says, the crux of the matter is whose side of the boundary you intend to put the fence on. It is often very difficult (and contentious) to determine the exact - to the inch - position of the boundary. Title deeds frequently fail to contain detailed enough information but have a look at them, if you can.
In the very worst case, and if you can't get any agreement from next door, I afraid you may have to put in new posts, on your side.
Or, if you're not bothered about neighbourly relations, just put your new panels up anyway and let him take the matter further, if he wants!