I'm probably going to answer my own question because I ordered a set of ball bearings to repair the extruder and then walked passed my lathe and realised all I had to do was turn some brass bushes the same 10mm od and 6 mm id to replace the ball bearings. 30 minutes later the printer was back on line. Now thinking about it I wonder why ball bearings are used. There is no critical clearance between the filament and the drive cog as the filament is spring loaded against the drive. Thus as the bearing wears a little overtime it's only a matter of increasing the tension. There would be no sudden collapse in the middle of a print job. Of course there might be good reason to once every 100  operational hours to re-grease the bearings. There would be plenty of warning of impending drive failure and plenty of time to figure in a new bearing. I expect however the brass bush to last the life time of the printer. 
Has anyone been down this path with any sort of success?