Mark,
There is a long rod that connects the vertical gearchange shaft to the lever on the gearbox. The long rod should be in 2 pieces with a clamping block so that it can be adjusted. Undo the clamping bolt - put the gearchange lever into 1st (forward position) - by hand, do the same with the lever on the top of the gearbox (rotating the back wheel at the same time, to make sure the gear is fully selected) - then tighten the clamping bolt. Both gears should now select correctly, but you may need to make further fine adjustment to ensure you get neutral when the lever is in the middle. Selection of neutral may be a bit hit and miss if any of the pivots are worn.
Regards,
Eddie.