Behringer X1 Infinium optische fader
After Some checking of the circuit diagram and seeing experiences of others online, I decided to try cutting the cable and reversing the outer connections (positive supply and ground). Fader worked fine afterwards!!! Really lame of Behringer, seems some ddm4000 mixers have the x15 connector soldered on backwards requiring a modification to make the fader work. Perhaps they could do the decent thing and acknowledge this and offer an adaptor instead of wasting everyones time!
Once working the fader is great - shame it needed modification to work as advertised in a ddm4000