You need to look at your deeds, or if your property is registered with the Land Registry, you can download the plan for a small fee.
There should be T markings along the boundary, the T lies on it's side, and the 'leg' protudes into the property responsible for the boundary.
If it is H markings, you are jointly responsible.
http://www.esriuk.com/industries/subindustry.a sp?SubID=22&indID=16
If it is your fence you can do what you like with it and don't need to notify your neighbour. Your neighbour should ask your permission before attaching anything to his side of the fence or painting it. It is neighbourly to inform your neighbour of your intentions though, especially if they have pets or small children.