Hi, thanks for your question. My name's XXXXX XXXXX I'm going to assist you with it.
There is no legal obligation for a landowner to cut back vegetation overhanging a neighbouring property unless it causes damage sufficient to amount to a legal nuisance. Nevertheless, in the absence of nuisance, the affected landowner can exercise common law rights to cut the vegetation back to the boundary. Having done so, the material that has been removed remains the property of the owner of the vegetation and, technically, needs to be offered to him (as it could have value - i.e. fruit and/or firewood) .
However, once it has been cut, the owner is not obligated to accept it, leaving the neighbour responsible for its disposal. Placing it over the fence, without his consent, could be considered a trespass (which arises under the law of tort). Further, if placing the material on his property caused any damage to plants this could amount to criminal damage.
It is usually considered pretty unneighbourly not to arrange for the disposal of vegetation overhanging your neighbour's property but it is not a legal requirement and putting the clippings over the fence is ill-advised, particularly if the neighbour has already advised that he intends to be difficult about it.
There is a lot of conflicting information on the internet regarding this and I'm sorry if this is not the answer you had hoped for.
Can I clarify anything for you?