Hi Adrian,
Sure sounds like limp mode,my previous A8 went into that mode, but that was due to the brake pedal switch being faulty,As we need to depress the brake pedal to engage the gear stick,thats when mine used to go into limp mode,not every time but occasionally.So not gearbox related as I had assumed,I think we always think the worse,with the well documented gearbox issues.
Needs to go on the computer or vagcom really
Rich