Two things I have realised since sending the initial post:

1/. In the circuit diagram, U3 should be 7805, not 7812
2/. I forgot to mention that my feedback is gained from a photo-interrupter/encoder wheel configuration mounted to the motor shaft. This feeds an LM2907 - Frequesncy to Voltage chip. The output from the LM2907 is adjusted to give 0-5V (stop - full throttle).

Cheers
Barry