So good news and bad news. I got cold feet over going straight to 15v, so bought a few trannys of different volts and started with 6 + 6 which was a no-go. 9 + 9 = no-go
Then at 12 + 12 and the controller would run the motor, but only for a moment.
I then tried the 15 + 15 and same result, the controller would only jog the motor.
No amount of twiddling IR or Imax made any difference, seems the current feedback circuit had a problem.
Bridging out the load resistors made no difference.

The bad news is obvious - couldn't get it to run properly, the good news is that a 15 + 15v transformer is probably correct.
Cheers