Ooh you've had no replies! Sorry I'm not usually in this bit. If you have a waitrose have a look for the duchy water from Scotland - it's mega low calcium at about 3mg/L. But even their own brand gallon sized bottles of 'essential' have only about 30mg/L. It's the calcium content that you're looking for. 'Soft' tap water (low calcium) is about 60mg/L or less. We have hard water round here at about 300mg/L which makes for a scummy kettle. You can find your water hardness online. Water filters can reduce Ca to about 10% of the input. Lots of bottled 'mineral waters' actually have very high calcium. But they should all be labelled on the back as to the amount of things in mg/L. The lower Ca the better. Have a nice break!