Cheers for that, how often do you think you'll have to replace the resin?
No idea yet. The harder the water, the more frequently you have to change them. I was going to just use it for final rinse but someone else who has a similar one, advises using it for the full wash phase which will increase the need for refills.
I might (if it's not too much of a faff) just use it for final rinse in winter or on overcast days, and use for whole wash in summer or on very sunny days, as it's the direct sunlight that causes water too dry too quick that's the main problem.
Anyway, I will keep tabs on the usage and let you know how long it lasts over this winter till I have to buy a refill if that's any help.