@AJP cheers for that mate. Yeah they're 3 1/2 years old which isn't old enough to expect that to happen.
We're actually a KW dealer at work so I spoke to one of the main guys for KW UK (about 6 times in 48hours) and we ended up at 25% discount and if I send the broken spring back they will do full analysis on it. Bollocks deal really but their perspective is they have no idea what stresses and strains the spring has been subjected too and hold no responsibility unless it's proven to be a defect.
There's probably a cleverly-worded disclaimer of sorts hidden away in the little booklet you get in the box, that makes it easy for them and hard for the consumer, with this kind of thing.
I suppose 25% is something though.
The main thing - you were crawling when it happened, and you and the rest of the car are ok. Can't imagine how I'd deal with a front corner suddenly dropping at speed