I asked this question a while back.
The answer I got was that the creaky bits are probably in fact the tension bolt and nut, and I needed to oil them.
This is easier said than done - you can put oil on the bolt threads, but it doesn't creep inside the nut, where it needs to be to stop the creak.
The solution in my case was to loosen the nut (carefully counting how many turns I made), oil the bolt, and then tighten the nut the same number of turns, thus pulling the now oily bit of bolt back inside the nut.
Problem solved.
I can now worry about all the other bits of the bike that are creaking. (Or is it me that creaks?)