Among them, the team of undercover reporters met and filmed a man who called himself Vicki.
He was careful about his security - moving the car in which they talked away from CCTV cameras - but open about the fake documents he could obtain, and boasted about customers as far afield as Sheffield, Bradford and Coventry.
Vicki said he could get people into the country on lorries, known as donkeys, organised by what he called his "man in Paris", and told how he could provide a fake "original" passport that had been "checked" to beat security at a UK airport.
Secret filming of BBC reporter's meeting with Vicki
When Vicki was later confronted with the details of what had been filmed, he denied doing anything wrong and said it was a case of mistaken identity.
WHO ARE THE 'FAUJIS'?
Punjabi term colloquially used to describe illegal immigrants
Directly translated as a private in the army, one who is away from home
Faujis are mostly poor farmers from the Punjab in India
A common route in is via Russia, Eastern Europe and France, stowed on the backs of lorries
Others outstay their visit visas
In this hidden world some "faujis" - the term used by the illegal immigrants to describe themselves - try to get by without any documents.
Others will have cheap, fake documents, and some will pay good money for original passports, for bank accounts, a Home Office registration card or for stolen identities on driving licences.
Documents are important because they provide a sort of legitimacy. What the faujis fear is simple: being caught and sent home. But with the documents they can get a bank account and do some work.
The undercover team found there was no shortage of job offers, including at a Southall chip shop where a fauji told of being employed for 12-hour days, six days a week at £150 - about £2 an hour.
One reporter, Mohammed, went there for work. The owner, Bhupinder Singh, said to "never mind" the fact he had no papers, that he would "handle that issue" and that the reporter should not mention it "otherwise you may be nicked".
After a 14-hour day with no break, Mohammed claimed he had another job to go to and asked for his day's pay, but Mr Singh refused, saying: "You don't get paid for two weeks, right?"
Mr Singh has told the BBC that he does not employ illegal immigrants, that all his staff have the correct paperwork and permission to reside and work in the UK, and that he did not pay Mohammed because it was a training day.
Another time Mohammed went to an area well known to Southall faujis, where they wait at the roadside to be picked up for casual labour. A man approached and a job "interview" was conducted.
"Do you want work?"
"Come on then."
Mohammed was taken to a building site and, without being asked whether he had any experience, was put to work on a roof parapet with no training, safety advice or kit. He was paid £35 for 12 hours' labour.
Time and again the importance of having the right paperwork was clear. Showing how easy it was to get hold of it, the undercover team filmed another fake document supplier, Anil Kumar.
Covert footage of undercover reporter's encounter with Anil Kumar
When Mr Kumar was later confronted, he also said he had done nothing wrong and that it was a case of mistaken identity.
This hidden community is an open secret - landlords take on tenants, employers want cheap and uncomplaining labour, while the criminals trade in people's lives.
Once the undercover reporters infiltrated this closed community, it was clear how easy it is to remain invisible.
One clear fact remains: the scale. In just one square mile, hundreds of illegal immigrants, scores of multiple-occupancy houses, and people dealing in fake identities, employment and fraud. A criminal network that is out of sight.
The undercover reporter's name in this story has been changed to protect his identity.
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