I did it recently... it's not too hard at all. I got the instructions from
www.northamericanmotoring.com (you have to register then search the exterior mods forum. I just had a look and the instruction page seems to be missing
I'll post what I can remember but anyone else who has done this feel free to add the bits I have missed
Have a max of 1/2 tank of petrol when doing the job. And don't smoke
firstly you take the cover off of the existing cap by opening the door gripping the cap firmly and twisting it off clockwise. Take off the actual petrol cap by disconnecting the rubber cable.
tape the surrounding paintwork with masking tape for extra protection then slide a credit card (I used my Tesco clubcard - no bumpy numbers) in between the paintwork and the filler cap at one of the three bumps you can see inside - these are where the clips are located, and prise out with a screwdriver. Don't push the screwdriver in far, just a little and work you way around the three clips pulling a little more each time.
It will eventually pop loose then you have to tug it out. This can be a bit tricky but isn't as hard as some guides make out. Just keep tugging to release the rubber neck that holds it in place. You will also have to bend a metal clip to get it loose so just keep pulling until it comes off (ooer).
Fitting is fairly obvious just work backwards. You may have to bend the piece of metal back if it came too far forward but this will be obvious when you try to put the new filler cap on (it won't fit flush if the metal clip is too far forward.
That's all I can remember... hopefully other will fill in/correct me on anything that seems missing/wrong.
It's a thirty min job if you take your time, but it's sooo worth it.
Good luck
PM me if you need further advice mid-job
J