Rob. When touring I'll usually have an "Andy Strapz" "Stuff Sakz", containing my sleeping mat and sleeping bag, strapped across the back seat and held in place with a couple of click buckles. I've never had anyone interfere with my bike or my gear. Helmets are different matter, as they can be worth big quids, and are easily nicked. Use a lockable cable loop to secure it to a suitable spot on the bike when you leave the bike. However, I wouldn't bother when going into a servo when refueling.