Our Mains comes in at the tap end of our bath ( well out of easy reach )
Then it splits and one goes upstairs to supply the rest of the house whilst the other goes to the kitchen.
I can't think of any easy way to fit something that size in the system.
It's looking more and more like I just have to accept that shower heads need cleaning every 1 or 2 weeks and shower units need replacing every few years ( still cheaper than salt )
Thanks to all of you for the help.
And thanks for saving me time, effort and money.
It looks like I'm gonna give it a miss.
Jim