One thing to be aware of, Hamish, is, I believe, that the searches are not returned from one place which is why you can get different results from the same search even on the same machine seconds apart. There are various different groups of servers returning results and it depends which one is returning the results.

As they are quite shrewd I'm sure that they return ads that reflect your browsing habits.

They are also very secretive. The company I work for have been doing some work with them and you can ask questions up to a point and then the shutters come down and they go 'that's how it is...' and that's your lot. Very entertaining.

The one thing that I do know that it does not take a blind bit of notice of if a site changes or not. I have a site that has sat in number 1 position on google in a category of 4,500,500 results and nothing at all has changed in it for something like 5 years.