Hi Colin.
Most of the tips have been covered here already.
Here's a couple more to slow the bar stewards down.
1/ Attatch a couple of personal attack alarms to the van and having replaced the cord with fishing line for better invisability, stake the cord/line to the ground. When the van is moved without invitation, the alarms go off making a right racket.
2/ Make a couple of drive posts from box tube steel get two sizes, 40mm and 50mm square. The smaller fits inside the larger one. Make up a socket from the 50mm box to sink into the drive. insert the smaller post and drill a hole through both to suit a long shank pad lock. Obviously you will need to fabricate a shroud to block the use of blot cropers on the pad lock.
Now fill the inside of the post with wet concrete mix.
The concrete mix causes the oxy acetyline cutting torch to blow back (make loud bangs)when trying to cut the post off.
It might sound a lot of work to make these posts but if it buys another 30 mins, that might just be enough time to force the thief to buzz off to easier pickings.
The trouble is, the thieves are well equipped and organised, so you have to think like a thief to stop them getting your goods. But do bear in mind that rather stupidly our government has give them rights to "health & safety" and the ability to sue for injuries while on your turf.
Of coarse make sure you have a wood chipper on hand capable of shreadding base ball bats and pick axe handles, after all, if they can't find the evidence and all that!!
Regards
Steve L.