To add a new mystery, the drivers side wiper (separate motors) is only wiping as far as tdc of the windscreen? Is there much can be done inside the motor or is it better just to replace?
Have you stripped the gearbox? It's a bit messy, but you will be able to see if there is excessive amounts of wear on the drive spindle and gear teeth ... or even if the gears are slipping.
I can't remember what I used on my Series I, but it was a standard bit off something else, as the wiper gear spindles were pretty much standard on most vehicles.