Hi,
I can discard the Thrust washers of the crankshaft, my engine was rebuild few time ago, about the release bearing car is working with a heavy duty roller bearing so for me the problem is inside the gearbox may be the layshaft and/or some bearings. In few days I will take off me engine and gearbox for inspection, I have the workshop manual but can I find any tutorial with fotos or in video? just to be sure 
Thanks again