Found out what I was doing wrong, I expect it will be ok now thanks

Thanks for the suggestions, which I have just tried. Unfortunately they haven't made any difference.

Having set the near focus then using the arrow keys the focus keeps returning to the near point, and the histogram moves from side to side and back again.

It's a shame because I really wanted to ...

I can set the nearest focus point just like it shows in the video tutorial

https://www.youtube.com/watch?v=3LDSGWPNe3o

but when I attempt to set the far point the camera seems to respond for a moment but then returns to the near point. Any suggestions to fix this would be welcome.