There are 2 different legal issues and you must deal with both of them.
The title deeds is a law of property issue, which seems to mean your neighbour has an easement preventing the placing of a window. I'll leave other lawyers to comment on this, but I believe that its more than a letter from the neighbours that you need.
The PP issue is part of the Town and Country Planning Act. Irrespective of the first issue, you may have to apply for PP, unless the window is on the ground floor (when you won't). If the proposed window is first floor and overlooks the neighbours house or rear garden, you may have to apply for PP. Ask Development Control at the Council for an informal meeting - they will give you advice for free. Getting PP (if required) doesn't remove the land title issue.