I have a similar problem, although not quite as severe, where I can turn the truck off but the key won't turn the last notch letting me pull it out completely.
I've found out this only happens when I park on the uneven gravel section of the driveway, on a slight slope and with the tires at different heights because of the bumps. the truck goes into park but it's like it doesn't go all the way and holds on to the key as if it was still on. Usually releasing the brake pedal and letting the truck settle completely and shifting a bit from Neutral to Park helps it "settle" and the key comes right out.
Dunno if this is what's afflicting your Disco but beware the dealer saying your ________ is broken, it'll cost you more than your truck probably did to fix
