It may be a sideshow to the Ken versus Boris battle, but the elections for the London Assembly will go a long way to determining the effectiveness of the next Mayor.

The 25-strong body has the responsibility of scrutinising the Mayor, passing his budget and providing key members of the new administration, such as his deputy.

But the body is set for its biggest overhaul in its short life - it came into being alongside the office of Mayor in 2000 - as some members retire and others face probable defeat.

"We could see up to 10 new faces," said managing director of London Communications Agency Robert Gordon Clark, who is to publish election predictions with London government expert Tony Travers.

"The Conservatives could take 10 of the 14 constituencies, which will put them in a much stronger position."

Earning in excess of £50,000 a year, Assembly members are elected either to one of 14 constituencies, which typically cover two or three London boroughs, or via a "top-up" list, which allocates 11 further seats, according to each party's strength across London.

But it means that if a party wins several constituencies, it is unlikely to get any top-up seats - and vice versa. At present all nine Tories hold constituencies while all five Liberal Democrats were elected from the list.

Labour has five members and the Greens and One London have two each. The five constituency seats most likely to spring surprises on 1 May are:

• Enfield and Haringey, where Labour is defending a 1,574 majority.

• South West, where the Lib-Dems believe they can unseat the Tories.

• Brent and Harrow, where a " threeway marginal" could result in Labour retaking the seat from the Tories.

• Lambeth and Southwark, where Labour hopes to hang on despite a longterm Lib-Dem challenge.

• City and East, where a sizeable Labour majority could come unstuck in the ethnic politics of the East End. Tory Assembly member Bob Blackman is defending the Brent and Harrow constituency - majority 4,686 - against Navin Shah, the Labour opposition leader on Harrow council.

Mr Blackman said: "We are going to gain Enfield and Haringey and we will have 10 seats on the Assembly. The Government is very unpopular but because of the way the system works, the more seats we win in the constituency section, the less chance we have got of picking up list seats."

Stephen Knight says he needs a 1.3 per cent swing to secure the Lib-Dems' first constituency victory by taking the South West seat from Tory Tony Arbour.

The Lib-Dems run two of the constituency's three councils - Richmond and Kingston - and hold three of the five Parliamentary seats.

Mr Knight said he planned a local campaign-based on "maverick" Mr Arbour's record. "It's a Conservative-Liberal Democrat battlefield. That means that whatever the Government or the Labour Mayor is doing is a bit of a sideshow."

Labour's chief whip John Biggs will be defending a 14,336 majority in City and East. He said: "East End politics are always interesting. You can never take people for granted."

BRENT AND HARROW

4,686 Tory majority: Held by Bob Blackman, it is one seat Labour could gain. Between 2000 and 2004 it was held by Lord Harris, the then Labour leader on the Assembly. It has several Labour MPs but Harrow council is run by the Tories while a Lib-Dem and Tory coalition, in which Assembly member Mr Blackman is deputy leader, runs Brent. He is also a parliamentary candidate for Harrow East.

CITY AND EAST

14,336 Labour majority: Labour would appear unlikely at first glance to lose this seat, but that fails to recognise the fear incumbent member John Biggs was said to have had in 2004. Then, however, Tony Blair was unpopular because of the Iraq war - especially in areas with large Muslim populations. The seat covers Tower Hamlets, the City of London, Newham and Barking and Dagenham.

ENFIELD AND HARINGEY

1,574 Labour majority: Joanne McCartney faces a tough fight to keep the most marginal seat in London. She won in 2004 thanks to Ukip denying the Tories more than 10,500 votes. Labour holds Haringey council, with the Lib-Dems dominating the more affluent west of the borough. There are no Tory councillors in Haringey. They control Enfield but they lost ground there in the 2006 elections.

SOUTH WEST

4,067 Tory majority: Maverick Tory member Tony Arbour faces a battle to return to City Hall for a third term. He is defending himself against the Lib-Dems, who have selected Richmond council's energetic deputy leader Stephen Knight. Mr Arbour led Richmond council until the Tories were ousted in 2006.

LAMBETH AND SOUTHWARK

5,475 Labour majority: Labour Assembly member Val Shawcross faces her usual battle to retain an inner-city seat where turn-out is low. Lambeth council frequently swings between Labour and Lib-Dem control. In Southwark, the Lib-Dems have consolidated their hold over the borough with the assistance of the Tories.

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