55 Tufton Street: The other black door shaping British politics

On a rainy afternoon earlier this month, Liz Truss walked through the famous black door of No 10 Downing Street for the first time as prime minister.

But under a mile away, there's another black door that's had a lasting effect on the previous decade in British politics - and looks like being influential under this administration too - No 55 Tufton Street.

The building houses organisations including the TaxPayers' Alliance and the Global Warming Policy Foundation - and is the former home of many others, such as Vote Leave and Brexit Central.

Just hours after Liz Truss made her first speech on the steps of Downing Street, she announced that her new economics adviser would be Matthew Sinclair, a former chief executive of the TaxPayers' Alliance.

And a couple of weeks later, the new chancellor, Kwasi Kwarteng, delivered the most consequential financial statement for a generation, ripping up decades of economic orthodoxy.

He was pictured celebrating with Mr Sinclair - a man who made his name working behind that other black door.

The influence of TaxPayers' Alliance began in 2008, when the financial crash led to bank collapse around the world.

"If you didn't want that to happen in the UK, you had to get growth higher," says Andrew Lilico, chairman of Europe Economics and Matthew Sinclair's former boss.

"One way you could get growth high was just to get spending down and it might not be a very pleasant way of getting growth higher, but needs must in these kinds of circumstances.

"There was a TaxPayers' Alliance report called How to Save £50 billion, which to some extent breached the dike on where things were going. And very shortly after that, others all chimed in. So quite quickly there were proposals for cutting spending by £150bn and £200bn."

In 2010, David Cameron became prime minister and ushered in a new age of austerity.

The TaxPayers' Alliance was no longer a fringe group frustrated with the Conservatives' approach to the economy. Instead, they became a key public backer of the government's approach to the economy.

"The newspapers or the broadcast media would have a spokesperson from an organisation, it could be the TaxPayers' Alliance, it could be another think tank," says Nicky Morgan, a Treasury minister in the coalition government.

"As a minister, if you're going to advance a difficult or a controversial idea, it's no surprise that before you announce such a thing, what you try to aim for is that phrase 'rolling the pitch'. You've got people outside saying, 'this is what we need'. So when you announce it, one hopes that it's going to be well received."

But the organisations at No 55 had started to attract controversy too.

Many of them have a long-standing policy of protecting the anonymity of their donors, something the Lib Dems wanted to change.

The coalition government did change the rules on lobbying. But the BBC understands the Lib Dems wanted those changes to go further - and to include think tanks, which do not come under lobbying rules.

Few would suggest that David Cameron and his chancellor George Osborne were Tufton Street's natural allies - one senior member of Osborne's Treasury team describes the TaxPayers' Alliance as "a bit of a joke". But they were useful in helping sell those austerity policies to the public.

After the 2015 election, David Cameron pledged to hold a referendum on EU membership - and that's when the relationship changed.

Vote Leave, which would go on to become the official leave campaign, was originally based at No 55 as well. Andrew Lilico, who was Vote Leave's chief economist in the latter days of the campaign, says the think tanks there were natural Brexiteers.

"I think that they are people who are quite optimistic about what the market can achieve. And they're quite pessimistic about grand state projects.

"So the European Union, as a supranational, multinational body would be an iconic example of something that they would be sceptical about.

"Matthew Elliott, in particular, who's the chief executive of Vote Leave, comes directly out of that that setting. He was the chief executive of the TaxPayers' Alliance."

After the Leave campaign won the referendum, the fight shifted again. The battle over how exactly to define Brexit had begun.

"People thought that the referendum would be the end of it, and of course in many respects it was just the beginning of the argument," says David Jones, minister for exiting the EU from 2016.

"Vote Leave wound itself up so there was there was nobody there. A number of other organisations did spring up to fill that vacuum.

"And Brexit Central was a very important one."

Headed up by Jonathan Isaby, another former chief executive of the TaxPayers' Alliance, Brexit Central also ended up being based at 55 Tufton Street.

"So it became almost required reading for those who were on the pro-Brexit side of the argument," says Mr Jones. "Every day you'd check in at Brexit Central and see what they were reporting."

Boris Johnson's victory in 2019 - and his pledge to take the UK out of the EU's single market and customs union - was another huge moment for Tufton Street.

After the financial crash, once-fringe views on public spending had become mainstream - and now the same happened with Brexit.

The apparent influence made the argument around who funds these groups rear its head again.

But while privately critical of where the money comes from, the Labour Party hasn't made it a public priority to reform the rules governing this area of politics.

"55 Tufton Street shouldn't have any more influence than any other street in the UK," says Angela Rayner, Labour's deputy leader.

"That street seems to dominate particular policy and what's happening in government and legislation and it's not transparent enough.

"Labour would consult on the wider definition of what lobby groups are - so that would include what is currently known as think tanks because we don't believe that the definition is wide enough, but also around transparency around where their funding comes from as well."

The BBC did ask representatives from the organisations mentioned for an interview, but no-one came forward.

Labour may want to change the rules - but for now, that's not in their gift.

Instead, last week's financial statement seemed to confirm that Liz Truss is more aligned with the ideas floating around No 55 than any of the previous recent occupants of No 10.

So what sort of new policies might the government start to enact?

The TaxPayers' Alliance has had a long-running campaign to crack down on paid time off for trade union officials, including when Mr Sinclair was chief executive.

The new Business Secretary, Jacob Rees-Mogg, met the TPA in March.

The BBC has used a freedom of information request to discover that the meeting was called to discuss paid time off for trade union officials - something Liz Truss has now pledged to crack down on.

Still, no one can be sure exactly what will take place behind the famous black door of No 10 over the next few years.

But perhaps by paying closer attention to what's happening behind the other black door, we might get a good idea.

'The Other Black Door' will be broadcast on BBC Radio 4 at 20.00 BST on Monday 26 September, and will also be available on BBC Sounds.