The guy I bought my roof rack from gave me a set of springs he used to use with his dispatch. Apparantly they go inside your existing springs and offer extra support. He said he was usually overweight and used these to make the van higher on its arches so the cops didn't target him. Something to do with towing caravans officially apparantly. Something like this - http://www.camperlands.co.uk/coil-spring-assistors.html
Maybe they would be a cheap alternative to help your existing springs.
Hi
Problem is with those assistors is that they put stress on the rest of the remaining spring and will cause premature failure. These assistors are sold in the caravan shops to combat the tail dive when towing a caravan on uneven road and not to overcome overloading of the rear axle.
Another solution was to pack the spring with tennis balls - not the cheap ones from Morrisons.
http://www.airide.co.uk/home.php is the best solution, but;
the payload of a 1.9d is 815 and 1.9td is 900kgs (very early models are also 815kgs) and the 2.0 hdi are 900 kgs. (models before 2006).
Even with a full 500litre tank and 3 x 100kg guys and a full tank of diesel your payload is used up.
We always need to be careful with the stuff we carry in our vans as an overweight van, even if it doesn't look overweight will attract a penalty. It could be that our van isn't overweight in total, but if the rear axle is overloaded then a fine will be given.
Spruce