Easiest/best thing to do is probably get a new set from https://www.sgs-engineering.com
They're more expensive than ones off ebay etc but they'll last a lot longer and it'll be a lot less faff than getting your original ones re-gassed.
In terms of fitting they're dead easy (only 3 fixings per side) so if you can use a spanner/socket you shouldn't have a problem.
Make sure you've got someone to help you or something to prop the boot open though, when I did mine I couldn't believe how fooking heavy the tailgate actually is.
Lastly make sure you get the bracket at the bottom of the strut the right way up, seen lots of people fit them upside down which causes it to foul.