Just use your tap water mate it will be fine. If anything buy some campden tablets, break one in half and crush it between two teaspoons add it to your water, stir and leave it to dissolve. Then add it as and how you please.
Loving these 'how to' threads Clibit, very informative, thanks very much! Will give all grain a bash at some point, I say some point as I don't think (know) the good lady wife wouldn't appreciate me buying any more beer equipment. I'm only just getting away with it as it is!