Thanks for the replies.
@Ihoujin, I am running on a raspberry Pi4, and just using VNC from Windows, so it is running everything local to the Pi, so I can't see this being an issue. I did check to see if I could spot anything else that might be using the Camera, but could not spot anything, alos what would intermittently start using and then stop using the camera?

@maxthebuilder, I had fallen foul of not setting primary or guide to match the actual camera in the past when solving, but it is now something I always double check, or am I misunderstanding what you are saying?

I had three clear hours last night and spent two trying to work out what is going on with polar alignment. I did check the code base and the error is only raised if the camera returns "is busy", but I cannot work out why it would be busy (and for random amount of times)? Can anyone suggest how to log camera usage? Unfortunately the forecast for the next 7 days is cloudy, and I'm off to a star party on Thursday (Kelling Heath), so dead cert it wont be clear! As mentioned before a couple of months ago it worked fine, but maybe I have set something without realising.