Technically if theres already a wall, the wall is the boundry and therefore your fence is an internal fence.
For example if you raised the level of the wall to 6ft 6inch or 2 metres it wouldn't be the fence they'd say you had to take down is it. now logically you can't have two boundry fences. Same with entwisi, if you now put up a one metre high fence and moved your fence one metrw from the boundary and made a path between, how can that be a boundary fence.
You need to make sure of course because they no doubt have already made their minds up.
Someone must have also complained. some people need to get a life.
I think you'll end up taking it down or you'll get fined and charged for them taking it down.
http://dictionary.reference.com/search?q=boundary