Thanks chaps.
What's the betting I forget to line up the drain holes!
I can't find marinabrian's method - anyone got a link?
Actually, looking at
this pic, maybe it's the isolators themselves that cause the springs to snap - i.e. they're too hard and are forcing the bottom coil to try and uncoil itself against the rest of the spring, causing a stress fracture. When mine snapped (both of them) the break was exactly where the spring starts to wrap round the isolator.
Here's the isolator on its own -
https://rimmerbros.com/Item--i-RKB100970