I swapped out the resistor in the trackclip to a 1ohm 1/4W, this wont effect voltage but acts as a current limiter which is important for the rechargeables. This resistor is only valid when using 3 batteries with the LED's in series.
Here's the pics. The only change I'm going to make is fitted a small plug/socket for the cable so I can remove the clip if needed.
Battery Case :


Resistor change :

All connected and working :




Works brilliantly with alkalines, just gotta test with the rechargeables but should be fine.
I was originally going to have have the battery box mounted on the top but it was too top heavy and the headset would fall off as soon as you tilted forward!
The cable will be shortened once I've fitted the in-line connector.