Edgie,
Upgrading the springs on your van wont help, I used to be HGV driver, all that does is make your van look lighter and therefore wont get stopped. Also I think your rear axle has a sensor on it to compensate the braking, your springs wont compensate for that.
Just find your local weighbridge and go in with your kit and say half a tank of water, they will tell you what you can and cant do, they are very helpful if you ask them. They look at your 'plate' and tell you how near you are to the limit, think it costs about £20.00.
They are not helpful if you get pulled by the yellow jackets or the Police. As I think someone on here got done a while ago will tell you.
Murky