I used to prefer heavy water, but got fed up with Kirk Douglas attacking my house every time I filled a bucket.
Thames Water say three.The heroes of Telemark if I recall?
Back to normal water, not the radioactive stuff, there is a huge irony;
The further North you go the more the menfolk blab on about being hard and use the term Southern softies when in reality the South has the hardest of the hard water and the further North you go the softer it gets.
As for beer, Southern hard water makes a better pint.
However there is the mythical statistic that if you live along the route of the Thames that the water has "passed" through seven people already before you if you live in London. I'm lucky as I live upstream a bit and it has probably only been through four.