it appears vauxhall supplied your vectra with a faulty component, this was the small oil jet for the cam chains.
because the service interval is a stupid 20k the oil filter breaks down and clogs the small oil jet, this then starves the chains of oil and then they start to rattle before finally snapping :evil:
ask vauxhall to cover all the costs, it's a design fault and the chains are supposed to last the life of the engine (100,000 miles)