The joy of local elections is their sheer unpredictability and the idiosyncrasies of the results. As even a glance at the candidates in the London mayoral contest shows, we are not seeing clones of Gordon Brown, David Cameron and Nick Clegg rehearsing for the big one in two years time.

First, London. Anyone who claims to be sure who is going to become mayor is just guessing. Not only have the pollsters disagreed, but there are bigger margins of error than usual because of the large number of people, particularly the under-30s, not registered and the difficulty of getting at the views of some ethnic minority groups.

Moreover, all the elaborate calculations about whether Boris or Ken will benefit more from the second votes of supporters of other candidates assume that electors understand the system and then use their second votes rationally. By rationally, I mean voting for the two leading candidates in the second round. For instance, if you use your first vote to back, say, Brian Paddick, the Liberal Democrat, it makes no sense to use your second preference to back Sian Berry, the Green, since that vote will not be counted, and therefore is wasted.

In neither the 2000 nor 2004 elections was there any surprise who the top two would be on first preferences, nor will there be this time. In 2000 and 2004, the two top candidates won only two thirds of first preferences votes. But only about three fifths of backers of third or lower candidates in the first round cast valid votes in the second. Overall, between 17 and 19 per cent of second-choice votes were rejected. That is a big qualification to predictions about where Paddick and Berry votes will go in the second round. The answer may be nowhere.

Second, Wales. Roughly 1,200 seats, or three out of ten of those being fought nationally, are in the 22 Welsh county councils. Welsh politics remains very different: more than 100 councillors have already been elected unopposed and more than a fifth of councillors, especially in rural areas, are independents. But a sizeable proportion of the Tories' total gains may be in Wales, where they are fighting from a low base (less than 9 per cent of all councillors) and are contesting more seats.

Third, others. Even in England, other parties are likely to win at least 10 per cent of the vote, nearly double their share of a decade ago. Most attention will be on the British National Party, especially if it wins the small increase in its London-wide share of the vote from 4.8 per cent to cross the 5 per cent threshold needed to win a seat on the assembly. However, the BNP remains a negligible force in local government nationally, even though it is significant in a handful of northern towns and London suburbs. Last year it gained one seat for a total won of ten. In 2006, a good year for the BNP, it gained 27 seats, notably in Barking and Dagenham, but for a total of only 32. The Greens are far more important, gaining 17 seats in last year's elections, for a total of 70.

So nothing will be quite what it seems, not least because the first results after midnight will be atypical. Savour the complexities. The 2010 general election will look very different.

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