G'day Alistair,
The lever (gear selector arm) should be free but without any play (slop). The claw is spring loaded to return to central after movement. Check for chips in the claw teeth.
The box should be in neutral and the arm horizontal when replacing the inner cover. Try it with no gasket, just a smear of hylomar or silastic.
To get it back to neutral you may have to back out the cam plate plunger ( the thing sticking out the bottom of the box with a lock nut) a few turns. With the inner cover on, and a piece of pipe over the arm, push it down whilst rocking the back wheel. that should engage 3rd. Push down again to get 2nd and again (not as far) to get neutral. If that all works refit the outer cover and try with the foot lever.
Hope this helps,
Cheers