Rear Light Head Scratcher

Out for a shakedown on the S1, light switch on, front light/speedo/ horn all OK on both side light and full settings. Only problem is no back or brake light working. Turn light switch to off and then brake light works. Switch light switch to on again then front lights and horn OK but no rear of brake light.
Anything obvious or a starting point?
Cheers G
Anything obvious or a starting point?
Cheers G