Re: Innumeracy?
When you desalinate water for drinking, I'd imagine you'd add trace minerals back in, to bring it up to similar levels to soft (nancy) Northern water, rather than hard (real men's) Southern water.
Heart attacks are lower in hard water areas than soft, although I can't remember if there's enough difference to care about or not. But I'd imagine you'd want to re-mineralise desalinated water.
You're also right that de-mineralised water is bad for metal pipes. You tend to get pinhole corrosion, as it attacks them. Normally where you use this kind of water you use stainless steel or plastic pipe (or just put up with more frequent replacement).
It is a very interesting set of calculations though. On the con side, you've probably got higher pumping costs, planning difficulties with siting desalination plants, all that energy used, and of course running costs. Those membranes are bloody expensive, and have to be replaced quite frequently.
But the pro-side is also pretty interesting. You can do less environmental damage by abstracting less from rivers. You should have less problems with drought. Also there's limescale to think about. An awful lot of energy gets wasted on water heating, due to the massive inefficiencies that scale introduces. If you could make South Eastern water soft, then boiler plant would last much longer, and we'd have to use much less energy to get the same amount of water heating done. Plus less need for softening plant, chemicals in cooling towers etc.
Leakage is pretty much irrelevant though. Those leaky pipes have to be replaced at some point, or they'll fall apart, and it's silly to spend money treating water, to then lose it. Even if fixing leaks looks expensive, it pays off in the long term.