Re: no responses
Toscana, for what it is worth,
My 3.0 sportronic suffered the exact same problem as yours - stuck in 3rd gear, and not shifting. This is of course a default setting when something goes wrong with the gearbox set up. I read in a Citroen manual (they use the same 4HP20 gearbox) that the gearbox will remain in 3rd or 4th gear if (a) there is a communications error between the instrument panel and the ecu or (b) the ecu is at fault.
The fault was eventually resolved by cleaning the connections to the gearbox ecu (and the main ecu as a precaution) which were found to be wet and mouldy.
Good work by Alfa Workshop in Royston.
ps - sorry, I don't know where the fuse for the ecu is, but phone Jamie at Alfa Workshop on 01763 245 481 - I'm sure they will help.