A safety pin that goes in to the head stock and through the bottom yoke centre steam. One that can only be inserted if seering lock is fully engaged to allign the holes. Alternatively a device that goes via top yoke safety nut that you can turn so it expands and locks the top yoke to head stock. Option 2 while easier to exeute for all types of bieks would be less secure as they would be able to drill in to it from top.
I have a solution that works 100%: every time I park up I remove both front and rear wheels and the wiring loom and take them with me... ok, it means I have to carry a really big rucksack around but it has always been there when I get back
Probably not in many areas of London. Bike riders try the same thing and come back to just a padlocked frame.
