We used to have a van with a bulk head and the side door had a bar that was attached to it that went through the bulk head at low level and had a padlock on it.
Vehicle crime is rare in my area, I doubt anyone will be cutting a hole in the roof. The only reason it got done in the first place is because I left it parked in the car park of a block of flats for a few weeks. I'll leave it on the road from now.
I need something visible from the outside that will act as a deterrent, but also hold up to attack. A hook type deadbolt sounds ideal. I'm seeing prices of around £60-70. Anyone got one kicking about or know of a cheaper source?