Barry,
If you still have the flywheel off the engine, check that the clutch plate is being gripped by the pressure of the clutch springs - if it is , the clutch/flywheel is probably assembled correctly. Then refit the flywheel and check to see if there is any free movement of the operating cam with the cable disconnected. If there is no free play, the flywheel is probably going too far onto the crankshaft (probably due to previous attempts to restore the taper) and some reworking of the thrust race may be necessary to gain some free play (or, if there is enough clearance inside the flywheel, a thicker than standard clutch lining could be fitted - this would give more more free play). If there is free play but the cable wont reach the operating arm, a new, longer inner cable will have to be made. Regarding the thrust race - Douglas used several different configurations - on some, the thrust race ran direct on the face of the operating cam - others had a removable hardened track - some had a plain bronze thrust washer - others, like yours, had the thrust washer fitted with balls. Because of these differences, reference to another machine may not necessarily provide the solution.
Good luck - don't let it beat you!
Regards,
Eddie.