I thought I would update this to set the record straight and clear any bad reputations...
Since I posted the original post, I have had a number of discussions with John at AIR and he has indeed demonstrated great customer service and stood behind his products.
As you can see from the pictures, the fenders after some help from John (advice on how to best mount the fenders and get them to fit) actually do fit quite nicely. If this was a road or show car, I would have to do the normal fine tuning to make them fit perfectly. But for a racing car that probably will have a few kisses with other cars during the season, I am happy with the fit...
The other issue I had with the fenders was that although I specifically asked John before I ordered if the fenders would fit a 2001 Boxster S with the factory headlights, and got told without a doubt that they would - they do not

I had not seen any pictures with the fenders installed on a car before I ordered and as such did not see that they were to fit the later face-lift model lights (like the 996 turbo etc). I had several discussion about this with John as well, and whilst I am still not sure what the best solution is to this, he has made up a set of headlight sections from his bumpers that he has put in the post to me for free that I can have grafted into my existing bumper if I want and will refund me back some of my money that I paid for the fenders.
I know there are many opinions on what makes good customer service, and some expect maybe more - but I thought John managed the situation nicely and I am happy with the results...
Now I just need to decide if I graft in the replacement headlight sections to the bumper that John has sent me, or make some plastic covers that will fit as in the cardboard template in this picture. I will be running a simple LED light rather than full headlights.