After locking my caravan, I noticed that I had somehow managed to get the key out whilst leaving the barrel in a misaligned state. I couldn't get the key back in and most Internet sites were no help. So here's a simple hack using a file to fix it. Start by giving it a spray with WD40.

Step 1: Key

The key has a small protrusion that
keeps the key in the lock when you're turning it. Simply remove that protrusion and you're all set. It's best to have a spare just in case like me, but it works with either key now.

Step 2: File

File the tip with a small file until it's flat. This won't stop the key working, you'll just need to apply pressure all the time.

Step 3: Line Up and Turn

Line up the inner protrusion with the slot in the barrel and push the key in. Rotate till its back in place. You're done, it should now work with your unfiled or filed keys!