Than you very much Eric. I've never treated my water before other than a Campden tablet, but I had hoped that I would get away with CRS. I have pumped all this into a calculator elsewhere and have an idea what else to add.
I tested my Salifert kit (and my use of it) with the test solution supplied today and it hit the expected figure absolutely spot on. I hadn't read the table to see what quantity would theoretically be left in the 1ml syringe beforehand, so went in suitably blind too. I'm impressed with the kit. I did a test when I got the kit a few days ago (twice) and the CaCO3 came out at 102.5, which seems quite a fluctuation, though I'm pretty confident in my testing, as far as it goes. That would get it some way closer to the levels you point out the water company figures would suggest. FWIW, the Max/Min figures given for Calcium have a 30mg/l range in the reporting period, over 8 samples.
Yorks Water were quick to point out on the phone and in writing that the supply can change considerably at any time, so I'm certainly going to take a reading for alkalinity on brew days from now on. I may apply the CRS and bring other salts in progressively over a few brews.
By the way, if I treat with CRS, gypsum and other salts as required, should I still add a Campden tablet? Probably showing my ignorance there.