Don't think it's wise to assume anything where planning is concerned!
I can almost guarantee 'they' will find some reason to interpret things a lot differently than you.
You mention permission being granted in July 1986. Has any work been commenced within the specified time scale for commencement of the development mentioned, on the grant of the original planning permission?
If not, it may be elapsed and a new planning application required.
Unfortunately, it'll probably only be the planning officer who can give you the definitive answer though.