those clamps dont look very secure
There not mate. I got clamped by one of thoses types about 6 years ago and was so livid that I cut the bar-steward off with my trusty hacksaw that was in my boot and left it in a pile on the floor.. 
Got a call from the old bill a few days later and all I had to pay for was the chain I cut (£20). Was worth every penny IMO.
Ive done a similar thing err....Ron!!
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I was in a van at the time, a site van for a structural steelwork erection company. Theres no clamp in the world that you could put on that van that wouldnt come off with the kit in the back!!