Have you considered issuing (or threatening to,) proceedings against the planners, for harassment and malicious and vexatious threatening behaviour.
Sounds very much like they're "flying a kite", as the posts here indicate that there has been spasmodic use of the facility, from RAF closure to present.
I once had a similar problem....a repair garage and filling station on a prominent main-road junction.....no planning permission, as it had formerly been a barn and was turned to a garage in the '20's- so, established use...I eventually removed the pumps and let the storage-license lapse. a constant stream of petty , niggling issues with the local planning, culminated in them telling me I wasn't allowed to sell cars!...Displaying them was a distraction to motorists negotiating the junction....I was able to produce a letterhead from the business, which had antiquated verbiage - "Accumulators charged" "Shell motor spirit" -and the clincher...."Any make of car supplied"
Stuck it in his hand and warned that I would sue for harassment if they didn't get off my case....it worked.
a few years later, a fire gutted the premises. Council informed me, if motor trade activity ceases for more than three years,your "established use" will be extinguished I don't know where the 3-years came from and they were evasive when it came to proof.....
sometimes , attack is the best defence......if they are exceeding the remit of their authority, the threat of sanctions from their superiors,or being heldpersonally responsible, can have a remarkable effect on "attitude-adjustment"
good luck!