Thanks Paul.
Back focus is probably the wrong term to use. As you note, that will change between F7 and F10 as the FR is not parafocal. I found in an other forum that this is about
6.24mm of primary mirror movement which probably equates to my 20 turns of the focus knob.
MY problem (I think) is to do with a change a spacing between either the back of the scope or the back of the FR the sensor. This is supposed to stay the same says Celestron, but I found that the image is flat when in F10 and curved when in F7.
I'm thinking that the focal reducer is defective as it is not adhering to the specified spacing.