Right...Mike and I have spent the day working on the car, and here's what we've found:
Moving the gear selector around manually (1st-4th) works fine. 3rd and 4th did feel a bit crunchy at times (like the selection was being made in two movements). The display on the dash correctly registered the selected gear.
Changing gear using the sele system still missed 3rd and 4th. 'Helping' the actuator by applying a bit of torque to the selector didn't make it select the gears any better. In fact, it felt like the movement of the selector was fine.
Changing the lateral potentiometer didn't make any difference. We didn't try changing the vertical one, and I couldn't find a sensor reading for this one on AlfaDiag
Tried a new finger bolt. No difference. Actually, there was a difference - the bloody thing wouldn't come out again and happily rounded
So, with that, and the gearbox doing increasingly random things (instead of jumping to 5th when it missed 4th it would drop to a flashing 'N'), we opted to remove the whole lot. In the rain.
With daylight fading fast, we're just about at a stage where we can take the old box out. Hopefully fitting the new one will be a bit easier. No sign of any improvement in the weather for tomorrow though