Alan I too was shocked by the local Halford's price which was exactly as you have quoted. However there is something strange here as others who have recommended de-ionised water on another site where users are predominantly U.K. based have said that it is available in big volumes at quite cheap prices. Unfortunately I cannot recall the stockists mentioned. I had thought it was Halfords but if so I couldn't find any! Maybe a google search will turn up something although ideally it should be available fairly locally. Liquid is expensive when mail ordered.
However if this stuff cannot be obtained cheaply then it might be worth checking your process. I say this as I too live in a very hard water area(the kettle calcifies very quickly) and yet I have never experienced
calcium deposits. I use the Ilford wash sequence and then 1 min in Ilford wetting agent and into an air dryer.
What I think is important is to risk drawing the film between your middle fingers after the wetting agent and then hanging your film up vertically. I have never experienced scratches in so doing. I have a fan dryer which may help. A hairdryer on the cool setting may help if you don't have a drying cabinet.
The other trick is isopropyl alcohol with the wetting agent and giving the film a quick shake. I have never needed this but Les McLean swears by it.
I had always assumed that the NorthWest of England including Cheshire had relatively soft water and is mainly fed from Welsh water which is quite soft. Maybe not but I'd be surpised if the water is harder than in S Northants
Best of luck in your search