I carry alot of weight in back of car because of my work and car does get really low because rear suspension is compressed so much so could I fit stiffer springs to help?
Every uprated spring for the Honda Jazz that i have seen also lowers the suspension, so i don't think that is a good idea if you carry allot of weight in the back.
I'd recommend that you purchase and fit some Coil Spring Assisters. They are a rubber strip that sits between the coils of the spring to increase the spring rate when you need to tow something or carry heavy loads in the boot.
Here is a link to where you can purchase them from:
http://www.rsmmotorsport.co.uk/products/601/0/Coil-Spring-Assisters-/detail.aspxTo find the right ones for you, jack the rear of the car up so that the rear wheels are off the ground and measure the distance between the coils on the spring.
You can then purchase whatever size best suits your gap (18-25mm, 26-38mm, 39-51mm or 51-65mm).
Steve.