buy a gas strut off Ebay or if you want to pay over the odds, the Vauxhall dealership.
The old strut pops off if you lift the little black clips away from the body of the strut at either end while pulling the strut gently.
Someone needs to hold the boot open fully while this is done.
The boot is surprisingly heavy with no gas strut help so be warned !
Gas struts are around £10 to 14 for the pair on Ebay (false economy to do one only, the other will be on it's way out)