Solution found!

It seems the order that I needed was

Start XS++
Flash FS
Quit XS++
Start XS++
Customise FS with my own custom pack -
Quit XS++
Start XS++
Flash MAIN
Quit XS++

Now the phone starts up OK and I now have my K750 with R1CA021 and my required CDA number

Now I have tried that, I am now moving on to upgrade the K750 to W800 firmware!