I pulled the robot down, traced out the cables and reseated the right hand motor cable - all was then good.
The right hand motor connector is the green multi-pin. The left hand motor is at the other end of the PCB and is obscured by the interface daughter board.
Take care when pulling off the clear top cover to depress the sprung section in the battery container. Also take care with the copper/gold coloured flat cable going to the left hand side which looks a bit delicate. Other than that pretty straight forward.
Note that if you need to reseat the motor cables you should be able to do it without removing the battery compartment - I only took it out to trace the wiring.
Hopefully this will help others.
A big call out to Sphero. I have pulled apart many, many robots and this is a very well designed product. Nice!