I replaced the motor/gearbox units when they started making horrible noises.
I decided to dismantle the gearbox units and replace the bearings to see if that fixed the issue.
The above drawings show the gearbox unit after dismantling, the last image shows the constructional detail from US Patent 6029763
I managed to dismantle the gearbox without damaging any of the parts, with the exception of slight damage to the plastic on the input shaft bearing and I needed to drill holes in the main shaft oil seal, screw in self tapping screws so that it could be pried out.
I will be purchasing replacement bearings along with the mains shaft oil seal, the parts should be available from most bearing supply companies,
the part numbers of the original parts are as follows:
6904 RS, 6003 2RS, 6200 2RS, 6200 2RS, 6001 LU and TC 20 37 8 (known as 20x37x8mm Nitrile Rubber Rotary Shaft Oil Seal with Garter Spring R23 / TC)
Hopefully this information will be helpful to anyone else who wishes to dismantle one of these gearboxes.