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If the mayoral election was decided on a candidate’s grasp of policy, there’s a good chance Caroline Pidgeon would win it. The Lib-Dem hopeful — who has eight years on the London Assembly under her belt — has impressed even her better-known rivals with her inside knowledge of the City Hall empire.

Zac Goldsmith tells me “she’s great”, before glancing at his media adviser and asking, “Am I allowed to say that?” Sadiq Khan agrees that her handle on London policy is impressive.

But barring a political earthquake she doesn’t have much chance of winning London’s top job. The Lib-Dems are hovering at around three per cent in the polls and were badly shaken by the general election, which left them with a single MP in the capital. Many felt the no-nonsense Ms Pidgeon would be a safe pair of hands.

Her policy expertise has been honed during her time holding Boris Johnson to account — she has been one of his sharpest interrogators — and on display throughout the election campaign.

Ms Pidgeon catches the eye of commuters in her yellow coat when we meet outside the under-threat Black Cap drag bar in Camden.

In a cafe round the corner, she says she wants to “do” something for Londoners, rather than “be” someone, and is unafraid of criticising rivals, in particular Mr Khan and Mr Goldsmith.

“There’s a lot of cheap shots which turn off voters. I’ve seen better behaviour at my toddler’s nursery than I sometimes see at the debates between the two of them,” she says. “They are playing a very hard campaign and it’s already got incredibly dirty and I think it’s only going to get worse.”

How well Ms Pidgeon performs could help determine how many Assembly seats the Lib-Dems win.

She wants to make fares fairer — including an early-bird discount and a one-hour bus ticket — but has held back from promising a cut. She doesn’t believe Mr Khan’s fare freeze proposal is realistic.

“It would take huge investment out of London’s budget at a time when the Government is removing the £700 million a year revenue funding to TfL. Ultimately if we want to see the transport network growing then we’ve got to see investment in transport,” she says.

“I just don’t think Sadiq’s plans stack up. You will see fewer buses. You will see increasing overcrowding at all times of day on trains and Tubes. We won’t be able to afford the upgrades. It is really, really worrying. We need that investment in order to help to build the housing to grow the city.”

She describes London’s poor air quality as a “silent killer” which she would address with an immediate ban on dirty diesel vehicles in central London and switching taxis and single-decker buses over to fully electric as soon as possible. The congestion charge would go up to £20 at rush hour.

Her plans for the Met include 3,000 more police officers on the transport network to help tackle the problems of extremism — acting as “extra eyes and ears” — and also sexual offences.

She describes neighbourhood policing as the “best thing” the Met did in 20 years and suggests Mr Johnson made a “big mistake” by “stripping” local teams back to just one Pc and one PCSO per council ward. She would like them brought back to full strength.

She wants the Government’s much maligned anti-radicalisation strategy, Prevent, to be devolved to London government so it can work directly with community groups and young people and would encourage knife education in all secondary schools.

Ms Pidgeon has promised an “Olympic effort” to get 200,000 homes built over a four-year term including 50,000 directly delivered, with the money raised by retaining the £20-a-year Olympics council tax precept. She has proposed following the lead of several London boroughs by setting up an arm’s-length building company to increase borrowing power. “There’s only a certain amount the private sector is prepared to build because it’s all about profit margins. We’ve got to build homes on a scale that we’ve not seen for a generation,” she says.

She has placed a particular emphasis on supporting working parents. (She has a two-year-old son of her own). She wants to make it easier to build more nurseries, provide more wrap-around childcare and train more childminders in an effort to harness the huge economic potential wielded by mothers.

“When 10 per cent fewer women are returning to work in London that has to set alarm bells ringing when we have such a qualified workforce. We have to offer more flexible working,” she says.

As a Lib-Dem she is a passionate European Union “in-er” but isn’t sure if the referendum will affect the outcome of the mayoral race. “I don’t think it’s the issue on the doorstep, but it might affect how people transfer their second preference vote,” she says. If the race is close, her Tory and Labour rivals will both be desperate to get their hands on her supporters (in 2012 the Lib-Dems came in fourth on just over 90,000 votes). But she has no plans to endorse either. “It’s not my job to promote other candidates,” she says.

Does she get frustrated when they pinch her ideas? Mr Khan has already adopted her one-hour bus ticket proposal and Mr Goldsmith her plan for a construction academy.

“No, it’s very flattering. I’ve learned on my time on the Assembly that actually you can achieve things working with other parties,” she says.

But she doesn’t think her rivals have been radical enough. “You’ve got to be bold and have a vision and I don’t see that. They’re too worried about upsetting any group and want to please all people, and actually you can’t do that.”