You might be correct in that the planning officer is unaware of the condition. However, if you make a planning application, the officer dealing with it will have to trawl through the records and it will come to light.
Class A of the Permitted Development rules is a very broad area, intended to include mostly extensions to dwellings, and the restriction on your property is there to ensure that the development does not become too high-density, with little amenity space. But Class A also includes decking and it's difficult to see how it would be contentious unless it's grossly overheight and infringes neighbour's privacy. Even if your neighbour objects after you making a planning application, the final say is still down to the officer dealing with it, and in practice, if an officer recommends an application for approval, it generally gets approved regardless of how many neighbours object.