If you pull it back up, does it flop back down?
If you can't see any broken or seized joints, it would be worth taking the tin work off to get to the GCP lever on the top left of the gearbox (As viewed standing behind the ferret, looking forward) Check the joints there.
If no joy, try disconnecting the linkage from the bit attached to the gearbox and manipulating the gear change lever directly on the box. (This is what I ended up doing)