Tom, like you I'm in a hard water area (London) and now only use deionised water for making developer stock solutions and certain toners such as gold chloride. Simply to prevent precipitates forming.

For mixing working strength film developers I just use filtered tap water. However I've started to use Pyrocat-HD and I noticed a white precipitate formed when mixed with tap water. I cured this by using a good quality Brita water filter. A much cheaper option then using my deioniser, the ion exchange resin carts. are bloody expensive.