locking wheel bolt

So, travelled down to St Austell at the weekend which was the first proper run in the van. Got half way and pulled in to a garage for a drink. As my mate got out he heard a loud hissing sound, "I think you have a puncture mate" it was that loud we assumed it was split or something big. Anyway...

Not sure what the best way to secure my spare is. Currently I am keeping it in my van, but it is becoming a pain. I don't want to go to town on some homemade metal work. Is a locking wheel nut enough of a deterrent? From what I have read in the past, if the thieves are professional enough they...