To close this post: I eventually gave up on the original VFD and opted to buy a new Delta MS300 that has MODBUS communication. After some fooling around I got it up and running with proper spindle stopped, spindle at speed +++ signals. All well.
Top tip to anyone following the same path: MB2HAL does not support MODBUS ASCII (default on the drive). Need to change drive to MODBUS RTU. Would have saved me a couple of days of hairloss if I knew when starting out.