Many thanks for your quick and comprehensive responses to my question - much appreciated. Of course you are right with “rankdir” - I suspect I was trying just about everything and forgot to remove it before uploading.
I guess I have a choice to make regarding the use of “constraint” - either have the top edges of the clusters aligned or allow graphviz to reorder A[A-n], etc to minimise path lengths and crossing between them.
Again, thanks for your time and consideration of my issue.
James.