Halfords sell (manual) impact drivers - they come in a box with a bunch of end fittings and you just use it with a socket.

If I were you I would find a garage that has an impact gun(air powered) and go there with a 6 pack of beer and ask them to back it off for you.

You can then as mentioned above nip it back up with your own socket, dieve home then do the swap and nip[ the nut back up.

Twating your steering colum with a manual impact driver is not what I would do - although these are good tools I wouldnt say this is the best application for it.

Hmm.. that's a good point - whacking the steering column doesn't sound terribly healthy. I think you're right - nipping to a garage armed with a 6 pack does seem to be the best option, and could save me a lot of struggling. Thanks Jevvy.