An accident - am i liable?

An interesting issue. If the van was left with the main subcontractor then part of the answer may be in an old law called bailments. If he borrowed it for his own use he is liable for any loss. If it was left in his care for reward then he is liable in ordinary negligence and if just left with him he is liable only in Gross Negligence. He instructed you to move it so you were acting on thier instruction and had no cause to check the insurance situation. There should be no claim on you or your insurance policy and the owner and the sub-contractor may have to sort it out between themselves and their insurers. The main contractor should have contingient vehicle liability cover as part of his contractors combined policy anyway.