gear shifter
 

All gear gates engage with engine OFF
I can engage any gear and, then, start the engine. Then the car will move under its own engine power upon letting off the clutch.
BUT..... when driven on down the road no other gear engagement can be sellected.

Any ideas????

When driving, can you get it out of gear into neutral?
If you keep the clutch engaged and lift the throttle, can you put it in neutral?

Hopefully that will give an indication whether it is clutch or gearbox related.