The winner of the Ken and Boris show will get the headlines on Saturday morning. But that will be only part of the story of Thursday's local elections with contests in 137 English and 22 Welsh councils (although none in Scotland). It is quite possible that London may not speak for the country as a whole. Ken Livingstone could squeak home for a third term as Mayor of London, even though Labour has dreadful results and the Conservatives excellent ones in all other contests.

Outside the capital, this year's elections will not produce the dramatic shifts of recent years. The Conservatives could, and probably will, perform better than last year but achieve well under a quarter, at best, of their 911 gains of council seats in England last year, or the capture of 37 councils. This is partly because fewer seats are being fought this year: about 2,760 in England (and 4,023 including Wales), down from 10,500 last year.

Apart from the elections for the London mayor and the 25-member Assembly, most of the ballots are for thirds of seats in metropolitan big cities and industrial towns and shire districts.

This in itself limits the scope for big changes of control. Half the seats in seven English shire districts and all the seats in four with new boundaries are being fought, as is every seat in four new English unitary authorities (in Northumberland, Co Durham and two in Cheshire).

The other main reason there is unlikely to be a big turnover of seats and councils is that most were last fought in 2004, when the Tories did well and Labour badly. So there is less scope for the Tories to make further big gains.

Nonetheless, if the Tories do the same as in 2004, it will still be very bad for Labour, which even now has the fewest councillors for 35 years.

The key indicator is the national share of the vote estimated by the BBC from results in key wards and meant to show the equivalent position in the whole country. In 2004, the Tories won 38 per cent, Labour 26 per cent (a low only otherwise touched in 2006), and the Liberal Democrats an historic high of 29 per cent.

The Tories have, however, achieved a share of 40 per cent in the past two years, while their two main opponent parties have been in the 26-27 per cent range.

To judge by recent opinion polls, the Tories will hope to obtain more than 40 per cent in order to back up their claims of being on a path to winning the next general election outright. Labour won shares of 42, 46 and 43 per cent in the three years before its 1997 victory.

Conversely, Labour fears not only that its share will fall behind the Lib Dems, as it has twice recently, but that it could drop to a record low of 25 per cent. The Lib Dems would be relieved to be in second place.

In terms of seats, the Tories will be buoyant if they win more than 200 seats; quietly pleased if they gain 100 to 200; but would be very disappointed if they gained fewer than 100.

By contrast, Labour will be able to claim that the worst is over if it breaks even; relieved if it loses fewer than 100 seats; but expect panic headlines if it loses more than 200 seats.

The Lib Dems hope to gain from Labour in northern industrial seats but expect to lose to the Tories in the South. Overall, the Lib Dems are nervous because they are defending gains made in 2004.

The councils to watch are Reading, where Labour could easily lose overall control, and councils now with no overall control that the Tories could capture on a small shift of seats, such as Cheltenham, Vale of Glamorgan, Bury and North Tyneside.

The performance of the British National Party will be watched closely. It is contesting 562 seats, nearly a fifth of those being fought. The party still has fewer than fifty councillors across England and last year it gained seven seats and lost six. Its main hope is to gain the 5per cent of the London-wide vote that would give the party a seat on the Assembly.

Green candidates are fighting a quarter of the seats and the United Kingdom Independence Party is fighting one in seven.

Voters will have to be patient, however, because a third of the councils, including London, will not count until Friday and whatever the trends in the rest of the country, it will be the London mayoral race, declared early that evening, that will shape the media and political mood next weekend.

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