Hi Matt, If you take the body off and remove the 4 screws that hold the fan housing in place. Then you can pry the housing up and turn it 90 degrees and you will see the top of the bogie tower. The bit of black plastic with the small lump on it is the cover. It can be removed by putting a fine screwdriver down each side in turn gently and then it will pop off. You will see the worm gear. DONT lift the loco at this point as the bogie will fall out disconnecting the drive shaft, which happened to me and resulted if a fraught half hour trying to put it back in with needle nose pliers. The opposite end is easier.
I am going to try PJ10s advice. I'll let you know how I get on.