Thanks again for the info, appreciated! The forum is great for those of us who don't have a lot of experience!
I have a BMW warranty on it, which was transferred to me by the previous owner when I bought the Z - it's booked in next week to have some things done so I'll ask them to change the springs too. Might as well use & abuse the warranty!
I'll try to find another Z & have a look. In your experience, can anything else cause the sagging?