Dapol decoder function problem

[Oz63]

I just acquired a Dapol Class 29 with a factory fitted DCC decoder - I assume it is the Dapol Imperium 21 pin decoder. It all works great except for one lighting function. According to the user guide that came with the loco, F0 is ‘light engine’ mode and the front and rear lights should illuminate appropriatly in the direction of travel. This is not happening - in forward the front light comes on but not the rear red lights. In reverse the red lights come on at the other end but not the main light.  I can manually turn on/off the appropriate lights and all the lights work, but just not in accordance with what F0 is meant to do.  Any suggestions?