Just an update on this for anyone in a similar situation...
I opened up the light by twisting the inner silver bezel with a pair of long nose pliers then the light just comes apart. It turned out that the power cables had ripped off the PCB, one had pulled the surface of the PCB off and the other had snapped off at the top of the solder point.
I couldn't get a good solder join back on to the PCB so I have bought a new light for my mate.
So looking in to this a bit more it seems to be a problem others have faced though I don't think it's particularly common. The problem happens when you twist the outer bezel to select modes, sometimes the bezel catches the PCB resulting in the power wires ripping off so the advice is to select the mode and then leave it alone and disconnect the battery instead to switch off. It is also possible to select different modes by running a magnet (like your speed sensor magnet on your spoke) around the outside of the light which should activate the pickup inside.
