The important thing with gas struts is not to put oil on them, as doing so will attract grit and dust. Simply wipe them clean with a dry cloth. This is is good place to look for advice: http://www.sgs-engineering.com/car-boot-tailgate
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)
I'm sure if you get the struts and pop into most small garages / tyre fitters etc, they would swap the struts over for loose change, I certainly would.
You just need two people and a small flat screwdriver to lift the clips.
Make sure you buy the right ones. I had the same problem on a Vauxhall Cavalier many years ago; I got replacements and fitted them and the boot lid wouldn't stay up. I then discovered that my model, the CD, was the only one fitted with a rear wiper and so had more powerful struts than all the other variants.
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