Doug. wrote:
If the fence/posts are exactly on the boundary then the
position is not clear.

pk

******
All OK up until the last sentence.
If the fence is sited exactly between the two gardens and
extrapolates exactly to half width of the centre , or dividing
line between the two houses them it is general practice for the
fact to be written on to yours and his Deeds and it is general
practice to have the following sentence inserted , "Maintenance
of the fence is the dual responsibility of the two house
owners". So the boundary between the two houses is right down
the centre of the width of the fence, therefore each
householder is responsible for the upkeep of his half side of
the fence. The result is that neither side can interfere with
the structure of the fence or the siting of it.. They can of
course agree on the joint agreement to
renew the fence, in which case an agreement acceptable to both
will decide who does what and who pays for what. The new fence
therefore will be raised in excactly the same place as it was
before and at the height in general force at the time.
Doug.

You are correct for boundary fences that are designated on the deeds as
"joint" ie -|- on the plans.

But if the plans say " -| " but the fence has been erected " -|- " by the
non responsible owner. It is not, correction, I am not clear as to the
position

pk