I understand it was a gate guardian at Scampton, at the time the Lancaster was outside. Lincs County highways wanted to realign the A15, and the RAF agreed to move the Lanc inside the Gatehouse. Lincs said they would move the Grand slam. When they hooked up the JCB the bomb stayed put and the back of the JCB lifted into the air. Thats when they discovered it was still full of explosive. I believe it was eventaully moved to Shoeburyness range and detonated. If you check out the size of the crater here
RAF - Saumur Tunnel, 9th June 1944
its a good thing no-one drove into it!!
Other grand slam raids to be found here.
RAF - Grand Slam Raids
Ian