There are various different causes highlighted on here but mine is definitely the tilt adjust nut that kept coming loose. I nipped the 13mm nut up myself, with the guidance of the forum, by lifting the top half of the steering column shroud, which totally eliminated the knock/click, but it came loose again after a very short while. So I asked the dealer to look at it last week on the first service and when I picked it up they said they had tightened the same nut, it lasted 8 miles then came back.
I decided that because I knew where it was coming from, I would remove the bottom half of the shroud and get more access. I tightened two other bolts mentioned on the forum that hold the column to the cross beam, but this made no difference, the knock was definitely the tilt mechanism.
As you tighten this tilt mechanism's bolt it becomes increasingly more difficult to move the release lever to adjust the tilt so I decided I would set the wheel at my required height and forego adjusting it and tighten it right up. This seems to have done the trick, I've done about 40 miles since and the knock hasn't returned so far (touch wood). Of course I have no tilt adjustment now, but I have a silent car, no more knocking as I turn the wheel or when going over a rough road.