Got the cooler off. For those who will tread this path in the future, the nut can just be reached with a 10mm ratchet spanner. There are two pipes in the way; these need to be unclipped and tied back out of the way.
You are right about the corrosion. Where the egg- shaped pipe end touched the aluminium of the cooler, the ally has completely disintegrated- it looks like someone has gone at it with a hacksaw. I cleaned it up as best I could and renewed the screw thread, but I'll make sure it is a tight fit before putting it back on. The other pipe was also starting to corrode, so that was cleaned up and a layer of hammerite applied.
I would urge anyone else whose cooler pipes haven't yet gone, to have them cleaned up and coated with something at the next service, or do it yourself. You can get to the pipes by removing the wheel arch splash guard and don't have to take off the undertray.
I am used to old Alfas, mainly Giulias. What amazed me was the amount of corrosion on the underside- more so on a 5 year old car than on my 40 year old cars! I have just spent the last half an hour cleaning rust off and injecting/ painting waxoyl!