To cut a long story short, I found my local filtered water supply is not quite as clean as it is supposed to be and talking to many locals they concur with me. For the time being I am buying in spring water in. There was a time I thought the idea was a bit daft, but just shows how you can change your mind. I made a mistake renovating years ago by painting the corrugated roof of my house with a paint that although it was water based it was not for collecting drinking water. (See my gallery) However I did not see the warning until I had already ordered in the paint. What I plan on doing is replacing the roofing iron on the back extension with some unpainted galvinised roofing iron, like good second hand stuff. As for new corrugated iron, what is commenly used these days is Zincalume. This slightly more expensive than galvanised iron, but I am concerned about about thuse of zincalume as aluminium is used in the manufacture. Pure water seems a pretty complicated business!