Loosen the two bolts on the reverse lockout plates - just enough so that the sliding section only just slides. Push it fully forwards.

Carefully locate 1st gear, then second. This should push the sliding section back to the point where it will prevent reverse being selected.

Retighten the two bolts and check you can select all gears. Also check you can select reverse with the reverse lockout disengaged (Lift reverse collar on gearstick).

This setting may require minor tweaking, so adjust as necessary.