Duetto sagging rear end

The rear springs on our 2001 mk6 Transit are beginning to sag and I was wondering if anyone had either replaced the springs or added air suspension or helper coil springs ?

Just ordered a new set from Midland road springs, could be an interesting weekend.

Well, now riding properly. The ride has improved dramatically, I can only assume that she was riding on the 3rd helper spring when under load. She did need a new pair of longer shocks. Many thanks to Midland Road Springs and South West Autos in Bristol for their help and hard work !

I fitted a new front spring from Midland Road springs to my 97 Medallion about 4 years ago. Has not noticably settled since then

We seem to having a Ford spring session. As I said regarding the problems with the front sprigs on the legend you have proved what I said about spring specialists. 
The specialist trades are disappearing. During my time doing breakdown work the biggest challenge was finding parts for older vehicles. Once the time restrictions were put in place we didn’t have time to look beyond local suppliers for parts. 
Before I decided to part with our Duetto I had talked to a local spring specialist in our area and they agreed the front ride hight was lower than it should and said they could supply front struts and springs to correct the ride hight.