Just found this Paul after a search on the tinternet, may help.
Not sure if you can get the cogs separately, but when the adjuster worked loose on mine I did a 'quick fix' using the inside of a cable terminal block like this...
You just need the inside of one connector, then cut it in half so you have a short hollow block with a single screw in it. Slip it over the end of the adjuster rod and then use a pair of pliers to pull the adjuster rod out of the back of the lamp until it's at the correct height (line the beam up against a wall and compare it with the other side). Once it's in the correct position tighted the screw so that the rod is held in place.
It's just about do-able single handed, but easier if you can get someone else to tighten up the screw while you keep the light in position.