Image copyright Andrew Medhurst Image caption Andrew Medhurst, pictured with his daughter Lottie, says he's done his "fair share" of damage to the environment

Andrew Medhurst was a high-flyer. He earned a six-figure salary and had worked all over the world for the likes of Lloyds Bank and HSBC.

But he gave it all up to join Extinction Rebellion.

It was around Christmas last year when he suddenly snapped. He was designing pension plan policies aimed at encouraging young people to put money away for the future.

But after reading up on climate change, and as he reflected on the scorching summer of 2018, the 53-year-old came to the conclusion that pension schemes "looked almost fraudulent" because the effects of global warming threatened the future those young people were saving for.

He handed in his notice at Nest - the National Employment Savings Trust - and told colleagues he was leaving to campaign for the planet. His banking background made him an ideal person to look after the group's finances and donations.

Since then he has been arrested twice, run as a candidate in the European parliamentary elections (where he only collected 420 votes) and annoyed his 24-year-old son, who thinks his father could have done more for the environment by staying in his job. "He thinks I'm a bit of an idiot," Mr Medhurst says. "But we agree to disagree."

Not everyone is convinced by Extinction Rebellion's cause or its tactics. The movement has been criticised for wasting police resources, targeting the wrong people by blocking public transport, being too middle class and lacking ethnic diversity.

The movement has yet to notch up any concrete policy changes as a result of its actions, although it has certainly contributed to raising public awareness of climate issues.

Partly that's through highly media savvy protests involving red clothing, fake blood, pink boats and celebrities. But it's also through people like Mr Medhurst pledging to reduce their own impact.

"I've done my fair share of damage," he admits, having led a high-carbon lifestyle largely "out of ignorance". Nowadays he avoids flying and is not ashamed to be seen in frayed tops that have the odd hole.

'Radical transparency'

However, as a former businessman he also brings a certain gravitas to the matter. As leader of the organisation's finance team, he spends his time focusing as much on revenues as on revolution.

Image copyright Reuters Image caption There was a £1,313 insurance premium for the - bungled - fire engine stunt at the Treasury

The press has pointed to the way it is "raking in" money from members to support its operations and Mr Medhurst says the group has raised more than £2.5m this year.

Between the beginning of March and the end of September, gifts from large donors - those giving £5,000 or more - totalled £1.2m. The list of benefactors includes the rock band Radiohead, which gave the organisation £300,000.

Meanwhile, an online fundraising page has attracted another £1m from smaller donors.

And the donations fund a sophisticated financial operation, with two limited companies in the UK and plans to set up an international branch in Europe, which will pay for the group's activism around the world.

But wherever the funds go its policy of "radical transparency" will be maintained, Mr Medhurst says.

Payroll

Extinction Rebellion's site lists every single expense, from a £7,350 payment for coloured boats that were sent all over the country to a £1,313 insurance premium for a fire engine that was used in a bungled attempt to spray fake blood on the treasury.

But, perhaps surprisingly for an activist organisation, the group's biggest outgoing is its payroll.

Activists can claim so-called "volunteer living expenses" of up to £400 a week, which set the group back £130,000 for the months of June, July and August alone.

Image copyright Reuters Image caption Supporters marched along Oxford Street on a sixth day of protests in London

The aim is to make activism accessible to supporters with children to feed or mortgages to pay, Mr Medhurst says. It's a trust-based system, and supporters don't apply for expenses if, like him, they don't need them.

But protests are expensive in other ways too. October's events have cost the group around £1m, he says.

The expenses include £30,000 for hiring toilets, an electricity bill of around £30,000, sound equipment - like microphones and speakers - that cost another £25,000, while the bill to feed 20,000 activists three meals a day was about £50,000.

The group spent another £200,000 on things like leaflets and coaches to bus thousands of activists from as far afield as Scotland.

And then there's the hefty legal bill, following the 1,828 arrests of its activists (although only a fraction have been charged).

Another £120,000 went on "media and messaging" costs in October and it had put aside £70,000 to pay for so-called "regenerative culture", which includes providing "safe spaces" for activists that need to recover after being arrested.

Image copyright Getty Images Image caption Protests disrupted central London for a fortnight in October.

But of course it's not just the movement itself that is incurring costs.

The Metropolitan Police say so far this year it has spent £37m policing Extinction Rebellion's activities - £21m on the October protests alone - more than twice the annual budget of the Violent Crime Taskforce, set up last year to deal with a rise in crime on the capital's streets.

But Mr Medhurst argues those figures pale in comparison to the future cost of climate change and points too to what he says are the billions of pounds the UK spends supporting the fossil fuel industries.

"It sounds like a lot of money [but] another way of putting it is: how much are we going to spend if the government doesn't take the necessary action to prevent societal collapse due to climate breakdown?"

Spending on climate protests are a "stitch in time to save nine", he says.

And yes, he admits Extinction Rebellion doesn't always hit on the right strategy.

"I don't have all the answers. We're just doing the best we can with what we think has got a chance of working."