General election 2015: Seven of the key battlegrounds

There are 13 target seats in London

You can preview the story of the coming election in London by focusing on just a fraction of the 73 constituencies in the capital.

That's because the election is won and lost in marginal seats - those which are winnable, by the BBC's definition, through a swing of 5% in the vote. They are the close contests.

Most but not all of the seats below are marginal. Nearly all are targets, identified by the parties as those they have to win to achieve their overall nationwide electoral dreams.

2010 results and key seats

1 Brent Central

Sarah Teather was 29 - then Britain's youngest MP - when she won a by-election in 2003 for the Liberal Democrats at the height of the Iraq war.

She announced in late 2013 she wanted to move on. Labour claimed, naturally, it was because she knew she couldn't win.

Others put it down to her disillusionment with life in the Coalition, not least after she was moved from her position as children's minister in 2012.

Her successor as candidate is Ibrahim Taguri. In January, his party attempted to give him a boost in profile by making him the Lib Dems' 'race equality champion'.

For Labour, Dawn Butler is seeking to resume her parliamentary career and needs a swing of 1.5% to win.

The seat covers an area that is often described as the most diverse in Britain.

Since this article was published, Ibrahim Taguri stepped down as the Liberal Democrat candidate in Brent Central after the Telegraph made allegations he had accepted a donation that was potentially outside the rules. He was replaced by Lauren Keith.

2 Brentford & Isleworth

It was Labour-held with thumping majorities of over 20% in 1997 and 2001, but the story of expenses claimed on second homes is held to have done for Ann Keen in 2010.

Now the Conservatives' Mary Macleod faces a tall order in hanging on for more than one parliamentary term. A Conservative majority of just under 2,000 here would disappear with a swing of 1.8% to Labour's Ruth Cadbury, a Hounslow councillor.

Some might argue every time London's mayor Boris Johnson voices his suspicions that more Heathrow concrete is the Treasury's chosen way, it makes the coming task a little more difficult for Macleod. She and her opponents, of course, talk up the economic and social value of Heathrow Airport as a major employer in the area.

Sarah Teather, who won a famous by-election for the Liberal Democrats at the height of the Iraq war, will not stand

3 Carshalton & Wallington

Given the recent poll ratings, you have to start taking more seriously the possibility that the Liberal Democrats could lose this seat to their senior Coalition partners - despite a swing of nearly 6% being required.

The Blair revolution in 1997 benefited not just Labour. It also saw Liberal Democrats get a toehold in south-west London.

Tom Brake defeated Conservative Nigel Forman, who had been MP for 21 years, and he has held on ever since.

The Evening Standard reported on a leaked memo from Conservative chairman Grant Shapps 'urging' Tory activists in Carshalton and Wallington to campaign instead in neighbouring Croydon Central where Conservative MP Gavin Barwell is facing a difficult task to cling on.

4 Croydon Central

This is likely to be one of the most closely fought contests, with Labour needing a swing of 2.9% to remove Gavin Barwell, a former senior Central Office staffer, who won the seat from Labour in 2010.

The Conservatives lost control of the council to Labour in last year's local elections and their campaigners are quite open about the threat posed by UKIP who polled 17.5% of the vote then in the wards which make up the Croydon Central constituency.

If those UKIP sympathisers are mainly former Tories, and if they do not intend to come back to the Conservatives this May, then Barwell's position looks precarious.

The Labour party won back councils from the Conservatives in Hammersmith & Fulham and Croydon last year

5 Ealing Central & Acton

Having spent eight years as a London assembly member, watching the rise and fall of Ken Livingstone, Angie Bray knows that things can drop from your lap as abruptly as they land in it. She won in what was a three-way marginal in 2010, and the Lib Dem factor will be crucial.

An Ashcroft poll had her in trouble. An Evening Standard poll had her losing too.

Labour's candidate -who needs a swing of just under four percent to win it - is Rupa Huq, sociologist, writer and older sister of TV presenter Konnie.

What has added a more complex dimension to this battle is the recent apparent movement in support for the Greens. They have chosen Tom Sharman to contest the seat. He is the Green Party's national policy communications manager.

6 Enfield North

It was the demise of Michael Portillo in the ultra-safe neighbouring seat of Enfield Southgate which came to embody the 1997 Labour landslide. While that leafier constituency reverted to the Conservatives in 2005, the one-time Home Office minister Joan Ryan hung on in Enfield North until 2010 when she was defeated by Conservative Nick de Bois.

She publicly challenged Gordon Brown's leadership in 2008 and was sacked as a vice chair of the party. She said boundary changes counted against her, but there was also the matter of the money she'd spent sprucing up her then 'second home' in the constituency, at the time fully within the rules but inviting questions. She later paid back about £5,000.

But it has led to some wondering whether it was a wise choice for Labour to re-select her to try to win the seat back in 2015. That said, she must be favourite at this stage to restrict Nick de Bois to one term, given a swing of just 2% is needed.

7 Hammersmith

Last time round this was a key target for the Conservatives. They had been in charge of the borough council, promoting an aggressive cost-cutting agenda and lopping pounds off their residents' council tax bills year on year. David Cameron had let it be known this was his favourite council. However Labour's Andy Slaughter won with a majority of over 7%.

Then, Labour won back control of Hammersmith & Fulham last year after a campaign which focused on threats to local hospital provision, estate regeneration and affordable housing.

The demographics are changing rapidly here and huge developments at Westfield, White City and Earl's Court look set to entrench the area as a haven for professional middle class couples who long ago recognised they had no chance of settling in Westminster, Kensington or Chelsea. The Tories have chosen a local councillor, Charlie Dewhirst, to try to achieve the 3.8% swing needed to win.

• On Thursday, we turn to Hampstead & Kilburn, Harrow East, Hendon, Hornsey & Wood Green, Ilford North and Sutton & Cheam.