Good news, and thanks to everyone for helping.
I changed the two rear handles, and the front passenger handle, this afternoon. It took about two and a half hours to do them all, but a couple of problems cropped up.
The rear handles, as advised, were very easy to replace, and took about 40 minutes to do both. Unfortunately, I can't do a copy and paste to put a picture here, but the handles that I needed for a 2001 P38 have a small arm sticking straight out from the back of the handle, and a pair of long nosed pliers are needed to pop the lock mechanism arm off the arm on the back of the handle, and then just squeeze the lock mechanism arm back onto the new handle. As I was advised - easy peasy.
The front passenger handle was a little more difficult, but not much. The only thing was, I did need to remove the door card because the lock mechanism release arm has a "Z" bend head and I could not get it free from the mechanism on the rear of the door handle. I took off the door card and then I could see the release arm connected to the main lock mechanism, in the door, using an orange plastic clip. This just popped off by putting my hand inside the door, and then the handle could be removed.
The only thing was that the new handle didn't have a spacer that is fitted to the screw hole on the handle, and I had to persuade it out of the old door handle. And finally, the threading inside the handle hadn't been fully threaded in the factory, so it had to be tapped so that I could screw the fixing screw in fully. A bit annoying, but nothing difficult.
Fortunately, I had only one clip break on the back of the door cards - one on the top of the passenger front door, up beside the locking button, where there is a metal clip on the door card, broke as I was trying to get the metal clip inside the door frame, but I don't think that it will make any real difference as the panel is held firmly with the metal clip.
So all that I need to do now is fit the spring repair kit to the drivers' door. I think that after I fit the spring repair kit I will have to paint the door handle, because I don't want the hassle of buying new key fobs and then having issues with the metal keys being different etc. etc.
It is probably not possible - but could I take the existing handle apart and just replace the actual pull handle part ? One of my old ones is still pretty good as far as the colour is concerned but would it be possible to split the handle and put the good one in place of the discoloured one ?