I've tidied up the install significantly and switched the RFID tag to be on top of the robot. Previously it was at the front (to be close to the detection coil) but that was obscuring the charging coil and camera. Doing it this way also allows me to play with the balancing mode of the Jumping Sumo.
The charging appears to be very slow though so the next step will be to push the charging coil completely flush to the box to get the closest possible coupling.
Here's a close-up of the neater install (I've since routed the power supplies through a cable gland):
