Thanks for your response. Yes I've experimented with the dmesg command and I can sometimes get the Pi to recognise the UART but it doesn't appear to be communicating with the mount properly .
I agree it looks like a dodgy cable and I have ordered another .
Great work on your latest Astroberry image BTW ! The RPi 4 has definitely got what it takes for a much better astrophotography experience.
Cheers
Ian