I struggled for about 45 minutes trying to remove the old surround without loosening the filler neck. It is much easier if you loosen the filler neck first.
You don't actually need to remove the wheel, just jack the car up and remove the wheel arch cover. To do this you need to remove 4 plastic "scrivets" -- two are on the inside of the wheel arch (normally hidden behind the wheel), and two are to your right at the back of the wheel arch. Then also remove the two nuts (or is it three?) (up where the plastic cover joins the outer body panel), and one screw (under the car just in front of the wheel arch, where the plastic cover joins the plastic trim).
You then just lower the plastic cover and rest it on top of the wheel. This gives plenty of room to get to the filler neck.
The filler neck is held by two nuts -- one is just behind the fuel cap and the other is about halfway down the filler tube, fixing it to the innards of the car body. Remove both of these and the filler neck can be moved around enough to give you some slack.
This will free up the hinge side of the black plastic fuel cap surround, and makes it much easier to unclip the three clips.
Once it is all unclipped the only thing holding the surround in place is the filler neck which passes through a sort of rubbery seal. You will need to do a bit of pulling to get it off. It helps to retract the filler neck right back into the car body (I had a bit of trouble finding the right angle to do this, but it went in the end).
Fitting the new one is actually much easier (I gess these things were designed for fitting rather than for removal). Just re-fit everything in the reverse order of removal. Once you have the new surround in place and have re-attached the wheel arch cover, the trim ring just snaps into place. Lastly the chrome cap goes on.
The only thing I had trouble with was getting a good finish on the trim ring. It has a slightly roughened surface (for paint adhesion I guess). I sprayed it myself but couldn't get it as smooth and perfect as the bodywork. Others have had better results by getting a dealer to spray it for them (but this may not be cheap). There are kits on eBay with the trim ring already painted, but I don't know what the quality of the finish is like on those.