This is the figure you want.
Water hardness average: 21.5mg/l calcium
This is the measure of general hardness. GH can be given in any of several units. Your water company uses mg calcium per litre. Most fishkeeping resources use either mg calcium carbonate per litre, (also known as ppm) or german degrees.
21.5mg/l Ca = 53.75 mg/l CaCO
3, or in other words 53.5ppm
21.5mg/l Ca = 3 german degrees
Those figures, 53.5ppm or 3 german degrees are what you need when you look at fish profiles on here or other sites.
Your water company classes your water as 'moderately soft' but that band is between 20 and 40 mg/l Ca, so yours is right at the 'soft' end of moderately soft.
With that GH, it is extremely likely that you also have very low KH. I suspect that this is the cause of your pH problems. KH measures carbonate, which buffers the water against pH changes. Acids are chemicals which release hydrogen ions. We measure those with pH - pH is an upside down measure, meaning that the more hydrogen ions there are, the lower the pH. Carbonate reacts with hydrogen ions turning them into water and carbon dioxide. Once the carbonate is used up, there is nothing to remove the hydrogen ions so the amount builds up and the pH falls.
If you lived somewhere like London, they have very hard water, both GH and KH. They have so much carbonate that they never see a change in their pH. I have 3 german deg KH which is why I am in danger of a pH crash, and why it happened when I didn't do enough water changes.
I suggest you get the KH of your tap water tested as soon as your shifts allow, and once we know for sure that it is low we can work from there.
The good news is that your nitrate is 4.5ppm (or mg/l). You should have no problems keeping that low in your tank. Some people have 40ppm in their tapwater and they have trouble keeping their nitrate low.