To add to this one, I did the job today. Getting into it was easier than I expected, took about 40 minutes to get all the gubbins out working carefully and steadily, and about 30 to put it all back with equal care and plenty of white grease. The failed cable was the short one, nearside front and at teh front of teh door, rusted through where it goes round pulley, window failed "up".
In between getting it out and putting it back came 3 hours of wrestling, cursing and general unhappiness, same problem as others have reported with too much tension. I thought about cutting back the cable outer but didn't really want to, but in the end I did find a solution and am confident that next time it will take me a lot less time. So this is what I did:
Looking at the short cable, the end away from the motor, I cut back the black coating on the outer (but not the outer metal stuff itself ), about 15mm, makes it slimmer. Then using a small file and a knife I pared away at the slot on the side of the hole that the cable outer goes in to such that the cable can be pushed in sideways, kept it nice and tight, needed pliers to ease it in.
Then put it all together except for the fact the short cable outer is not in the right place, put backing plate in a vice, give the short cable a good hard tug or two and slip it in to the right place sideways, you don't have to pull it as far back as you would to get it in normally. The chance of it slipping out are minimal, but a cable tie through the convenient hole in the mounting plate ensures it never will. So you end up with the right length cable outers but can still put it together.
Photo shows it before the cable tie is tightened, white gloop is the grease not a nasty dose of fungus. All working nicely now.