Finally - success!
At the begining of this posting I asked a question about why I was getting a poor performance result for the Perfmon counter "Average Disk sec/Transfer" (while running the diskSpe.exe test)
I finally worked out why - I was running the test on a Windows VM built on an esxi 6.7 host. When I built the VM I used the "LSI Logic SAS" scsi controller. I changed it to "VMWARE Paravirtual" and ran the test again! Bang! The results of the test are now when they should be "average disk queue length" is around 8 and "Average Disk Sec/Transfer" is 0.011.
Totally forgot about the controller :-( - the time one wastes on blind alleys....
Oh well got there in the end!