Ok, as suspected I was missing something! Thanks Jonnymac! When I tried it before I stripped the motor, I think the clutch plates were all stuck from standing for so long, then I stripped the motor, clutch and gearbox etc. What I failed to do after checking and refitting the gearbox was to gently rock the final drive shaft whilst selecting a gear to help it engage!! Idiot!!!

With a gentle rock of the final drive output shaft all gears snick in perfectly!!!

Thanks again to Jonnymac for saving me from some serious head scratching! I can now finish the engine build and get it back in the frame.

Thanks again!