Going back to the OP, My tourer has had two new n/s springs. In only 50k miles of driving. As my car is currently off the road, awaiting some work, I am borrowing another 75 tourer. 3-4 weeks ago at about 2am, from my bed, I heard a loud bang... Yup.. the n/s spring on that car broke, and it had been parked up for 7 hours......
Guess the n/s wheel suffers more abuse from recessed drain grates etc which gives it more punishment than the o/s....