Hi Kezzy,
Our Doblo is also a 2002, 1.9JTD. A few years ago we had extra leaves made for the rear springs, by a company that makes truck springs. Result = far too hard.
The only way it was acceptable to drive was when loaded to the maximum.
We have since bought a pair of two leaf, heavy duty springs from a well known seller on ebay. These are much better and offer a reasonably comfortable ride unladen, while still easily handling max weight and towing.
I don't know what the rating is, but I will try to find out for you.
Sorry if this doesen't answer your original question, but I hope it helps.