Neighbour not built new fence on boundary line.

Boundary lines are always tricky as over the years houses change i.e extensions, conservatory, so when original deeds where drawn up the points of measurement change. If there has been a hedge or chain link fence then there is no exact boundary due to hedges growing and wire bending. If an existing fence has been in situ more than 7 years without dispute then this also becomes the boundary. The best way is to speak with your neighbour and agree the boundary prior to work commencing. Normally neighbours complain that they have lost land, but it looks like you have gained land, with regards to the placing panels on the fence your side unfortunately the fence belongs to the neighbour and you would need permission.There is no perfect answer unfortunately

Check the deeds and use your house and measurements as a reference point.

It takes planning, then agreement and action.

If there is a dispute on the boundary line, you would have to go to land registry and ask for a plan of the property and unfortunately measure it or get an independent surveyor to plan it out Depends on what council you are with as to say if the right or left hand boundary is the house holds. The old concrete chain link fence posts is normally the boundary line as they were put in when the houses were built, if these posts are in situ then you should be in titled to put a fence on your side.