You can see an analysis of your tap water if you download a full water quality report from your water company's web site. Well worth doing. It will show you figures for calcium, magnesium and a whole lot more. I see that you are in Birmingham so you possibly get your water from Severn Trent. Just go to their site and follow the links. Easy peasy! It would be very interesting to know what your calcium and magnesium figures are.
I’m in the Severn Trent region, and unfortunately they don’t seem to list a number for Magnesium content, just general hardness - unless it’s listed under a name I don’t recognise (I’m no chemist!).
This is the page where a post code can be entered.