My brother has a 2020 but mine is one of the older ones (HE) which was setup to use gravity fed water from the CW tank in the attic. Before we moved to France we had it changed so that it was an HF model (very easy for Kinetico specialists to change) to use mains fed water. No French house has a water storage tank in the attic!
I just looked at eBay and there are plenty of softeners there (be aware that a number of softeners come out of the Harvey's stable (Crown, TwinTec etc) and maybe less expensive. I see a new Crown for less than £800!
As I indicated earlier be wary of electric (time clock) softeners. Yes, I had one and it was constantly needing my attention. The twin tank softeners can be installed and forgotten, apart from adding salt blocks.
Do not be put off with some softener sales people saying that salt granules are less expensive than blocks. Yes salt granules are less expensive and, yes, I did use them at one time. But salt blocks, despite the added price, are so convenient. You can easily check (about once per month) to see how your blocks are doing in the softener, then add one as necessary. They are light to carry and the garnules tend to go all over the floor!! Mrs Frelon did not like that when I used to refill our older softener!!
Good luck!!