Gannet -- , Sep 12, 2002; 04:21 p.m.
I can't speak to the UK situation (so maybe I should shut up, right? grin), but I do know something about water purification.
The softened water Adams refers to is what you get after you run water through a water softener: big cylindrical thing, looks like a water heater, with special salts inside. In the US at least, this is normally only used in homes that have hard well water (there are exceptions).
Do you buy your "de-ionised" water at the local market? That's the way it's usually done in the US. Here, the stuff is sold as "purified water". This water is produced by reverse osmosis filtering, usually coupled with additional charcoal and possible resin filtering. It is actually better than consumer-grade distilled water for virtually all purposes, and the economics of its production have driven true distilled water from the market most places. If this is what you get, it'll be just fine for photo purposes.
Marine aquarists have to deal with this issue (which is why I know about it). Ask at the local specialist aquarium shop (not a "pet store") and they should be able to direct you to a supply.