Hi Martin,
So yeah, it looks like using ExFAT is the cause of the issue. I just got a 2nd drive and formatted it using Mac OS Extended (Journaled)… it didn’t give me an option in disk utility to use APFS.
However I’ve just done a quick test using the same project saved to the new drive and this time no audio drop outs 
So it looks like I’ve now got plenty of storage space to play with (4TB in total) I’ll look into moving all the stuff off the ExFAT drive and formatting that one to Mac OS Extended (Journaled) too, but for the time being I’m a happy Cubase user again!
Many thanks for your time and Support Martin, it’s been very much appreciated !
Kind Regards
Steve