I finally re-wrote my Sarkozy hub, because though it was about President Sarkozy, his many wives and much gossip about them all, the ads served up were dating ads such as "get a thai bride" (!), and clearly people interested in Sarko gossip wouldn't be interested at all in the ads (and hence no clicks).
I initially thought the problem was the use of the word "wives" in the title, but changing the title made no difference. Anyway, I then put the hub through a keyword density tool, which advised me that I had used the words 'married', 'divorce' and 'wives' a few times (each occuring about 1.5% of the time), as you would when talking about a man constantly marrying and divorcing. But these were what was triggering the "find a wife" ads. So I went through and used synonyms and reworked the text, and also played up the celebrity aspect of Carla Bruni to get more celeb type ads.
It was a success - the ads are now relevant at last (though the change didn't show up till I had cleared my cookies - Google's personalised ad cookie must have concluded that I wanted to see dating ads after I refreshed that page a few times LOL).
Note to self for the future: I need to use the keyword density tool when I create future hubs to make sure I've actually got the right collection of words dominating.