OK, I'm going to set it up as you suggest.
Looking at the existing cabling, I had Unicable from fixed dish, going to splitter and had two cables going to both Tuner A and Tuner B. That's why when I initially set it up, Tuner B said not connected (I forgot I did this, memory like a sieve) , and motorised dish going to Tuner C.
I'll give what you suggest a try, but don't understand why it wouldn't work with the way I initially set it up.