Hi Mark
We have just bought a Nemesis, and I agree they are heavy.
We haven't used ours yet as we pick up our caravan next weekend.
I spoke to the technical department at Towsure about the choice of wheel clamps when we were looking at buying one. The guy told me this is one of the best on the market at the moment. They rate it beyond gold sold secure. The guy told me that the heavier and more difficult it is to get on, the harder it is to get off.
As the saying goes though "If they want your van, nothing will stop them"