Ok so I ordered a water test kit. It's actually for aquariums but it test 6 different properties. NO3, NO2, GH, KH, pH, Cl2.
In the image the first strip is of Carrick Glen, the bottled water I was using initially. The second strip is Ashbeck from Teaco and the last strip is my own tap water. Amazingly, it looks like my own tap water is the best of the three.
The Carrick Glen has very high values for GH (general hardness) and KH (Carbonate hardness). It also has a very high pH value.
Ashbeck and my tap water are very similar. Ashbeck has a slightly higher GH value but everything else is very similar.
Think I might start bottling my tap water ala Del and Rodney with Peckham Spring! [emoji849]