Finally I have this solved. After an update to the Raspberry Pi 4 B, I encountered the usual issues with things not connecting or dropping out and my Atik 428 Mono just would not connect if I had the focuser plugged into the USB hub. After several hours of connecting one device at a time and swopping USB leads etc, I realised the issue was with the Addon 5 volt powered 7 way USB 3 hub. I switched the hub for an Anker 12 volt powered 13 way USB 3 hub, problem solved. Now I have good connectivity INDI finds all the devices without any input from me, there's less noise in the images and the line has completely disappeared. After so many reports on this forum about USB hubs and their suitability, it should have been the first place I looked.

I hope this helps someone else solve their issue.

Please note even if the hub appears to work from a connection point of view, it may not be working correctly for all your devices.

Alan