Cheers guys, some great suggestions here.

English -- there's one shave cream guaranteed to lather up like a dream in the hardest of water: Castle Forbes. Give it a go. You won't believe it!

Not sure if this is good science or not, but I might just have to run some basic comparisons now I've started this thread.

Rain
Boiled
Distilled
Mineral water
Guiness

Anything else to add to the list?