Ah-Ha! Thanks for this. I was aware of the 'temporary hardness' but not the reacion of borax with calcium. My water is very hard so even with boiling/filtering there must be enough dissolved calcium still present.
Only thing is distilled water round here costs one UK pound per litre! I thought of buying or building a device to make my own, but am uncertain how cost-effective that would be.
A vast improvement over your tap water would be to get the de-ionised water from the local garage or auto supply store. It's about half that price. Also used for steam irons. We pay about a couple of quid for 5L.