I always use the steering lock, but, frankly they're rubbish.
Then I add either a disk lock, although I'm told these are easy to get off, or an Oxford chain, which has the advantage of being able to link through lamp posts and the like, depending on which I have in the boot at the time.
The Downtown is a little large for a 14 year old to drive away easily without falling off, and it's also a little heavy to pick up with less than 3 blokes, so I'm fairly certain that if it was pinched, it would be a serious theft, and to be honest, there's no accounting for that so I've gone about as far as I can.
Alarms are useless, they don't scare anyone, and an immobilizer is easy to get round if you know what you're doing.
However, with only 28 in the whole country, it shouldn't be too difficult for plod to find it, if they go to the trouble to look out for it in the first place. What state it would come back in, of course, is questionable.