I know its some time since the last post on this thread but I had the same problem i.e. wiper movement became progressively slower before it ground to a complete halt mid wipe. When I looked closely there was no sign of motor turning the engagement plate, etc.
I followed most of Wee Mans post above except I didnt remove the circlip I just lifted rear window so it was positioned level and sprayed a good amount of WD 40 into the spindle. Next I used mole grips to move the spindle which was really stiff back and forwards repeatedly.
I repeated the process a number of times over a period of about 30 mins and the spindle eventually became easy to move. I reassembled the wiper blade and checked everything before flicking the switch and hey presto its working.
I was sure the fuse or wiper motor had blown but its working great (touch wood)