rear springs

I broke a rear suspension spring, about 1/2 hour to replace per side, I will do Both.
It seems that these rear springs are breaking not just on old Cars like my 92BMW 318IS Coupe, which has low mileage and is in great condition, but on New Cars also, we all know the Government don't take care of the roads, well I managed to source a good 2nd hand pair for my car; but as I made a temp repair, I also started looking for Heavier Duty/Stiffer Springs to try and rid me the problem;
I sourced a set of springs that looked the answer, asked the seller if she would measure them for me, WELL ARE THEY JUST NOT THE SAME SIZE, with one exception, the spring material is about twice the Diameter ; where the BMW SPRING AND IT IS A BMW PART, IS AROUND  3/8" or 10MM AT THE END SECTION OF THE SPRING WHERE IT SEEMS TO FAIL, The springs I have sourced are 3/4" or 20mm all the way through the full spring; The ones I managed to get were HVY Duty VW T4 van springs, which are far stiffer and do not give the same way the BMW ones do, with sports type vehicles, the stiffer the springs, the better handling, although I will be able enjoy better cornering now, I have two trailers I pull once a month for two days, so that will really make a difference.
I will warn you the actual length of the springs differ BY 3/4" OR 20mm SO IF YOU HAVE LOW PROFILES OR DONE OTHER LOWERING CHANGES THEY MIGHT NOT DO, CHECK FIRST.
I have no issues, as the car is Stock and gaining the extra strength/compression ability far out ways the 20mm lowering of the back end, and having looked through a lot of the BMW Forums, lowering back ends seams a popular thing.
I could add two plate 3/8" thick top and bottom to compensate, but I have sufficient clearance.
Cost £40.00 and an 1/2hour a side, a fit and young man or woman probably about 15 minutes a side if they are prepaired, and Replacing them is a very easy Job, not the way BMW advise but to do it there way would be a morning a side, and into a Garage.
Bye the way after I have done these repairs, so £200.00 including MOT COST and having only paid 250 for the car, and I use a Commercial Vehicle Garage for my MOT, and they go through the car very carefully, as they are normally only dealing with Bus's and Artics, and Heavy Vehicles, I was told do these repairs and The Car won't need anything for another 3-4 years,  It is in that good of condition. If I was not feeling too well I would have them do the work as I know I get a fair deal, but they do really check my cars out for me especially if it is one I have just got.
So if you are looking for an option on rear springs Size the Ones you have, then shop around for HVY DUTY not necessary from BMW but if you get ones to suit make sure you get the size, length and diameter at the widest point, and the diameter of the spring rod itself, You may well save yourself, either spending money on RACE Style Springs, or having Broken springs that in BMW seems to happen at a point which is difficult to repair, and also is a  weak spot.
Worth thinking about I am sure.
Jimmy