Alan,
I suppose it depends on just how far the one side is sagging as well as the cause. First, do a careful inspection to make very certain that you don't have a broken leaf. This is not uncommon.
If you generally drive alone, the idea of switching sides in the rear may be a valid one. If you decide on this course of action, get all new spring to axle mount pads and rear shackle bushings. Often, it's a good idea to replace the U-bolts as well. Many folks have a hard time getting these tightened just right, with the result being a "swimming" rear axle which steers the car in one direction when accellerating, and the other when decellerating. You do not want that!
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon