Unless there was a specific stipulation that 'time is of the essence' when you entered into the contract to purchase the part, Vauxhall are not in breach of contract.
However, as they've failed to provide the part within (what a court would regard as) a reasonable period of time, you have the right to cancel the contract and to demand your money back. You can then obtain the part, by the end of this week at the latest, from
http://www.breakeryard.com at a tiny fraction of the price you were going to pay to Vauxhall.
Chris