I'm sorry Bungle, but I have to disagree with you there. Detonation is a timing issue, and is precisely what the knock control system is supposed to be addressing. Pre-ignition should not occur in a naturally aspirated petrol engine unless something's seriously wrong. It is more likely to happen on a turbo engine, but again, only if something's seriously up the spout.
Detonation, while not as serious as pre-ignition, will gradually cause physical damage to the cylinder head. If you want to tell whether it's doing any damage to the engine, checking the condition of the spark plugs will give you some clues. If they look fine and the electrodes are intact, then there's probably not much to worry about.
The furthest I've got with looking at the problem is by using a diagnostics tool, I've noticed that the knock control system is retarding the timing on two cylinders more than the others. Now, I've done a compression test, and the (minimal) differences in compression between all the cylinders do not explain the differences in retardation of the ignition timing. I think the knock sensors are OK, because they were picking up excessive knocking on a particular cylinder and that went away when I replaced that cylinder's ignition coil.
I've tried swapping coils between other cylinders, but it's still the same two cylinders having their timing retarded by 5 or 6 degrees more than the others. The spark plugs are fine after 27000 miles, so nothing's seriously wrong, but something isn't quite working as it should.