Enoch Powell's infamous 'rivers of blood' speech at a Conservative Party meeting in Birmingham in 1968 marked both the end of his parliamentary career and the birth of an enduring mystery.
In that speech he quoted a letter which referred to the plight of an unnamed woman pensioner in his Wolverhampton constituency whose life had, he claimed, been ruined by immigration.
This, according to Powell, was her story: "She worked hard and did well, paid off her mortgage and began to put something by for her old age. Then the immigrants moved in. With growing fear, she saw one house after another taken over."
"She is becoming afraid to go out, windows are broken. She finds excreta pushed through her letterbox. When she goes out to the shops she is followed by children, charming wide-grinning piccaninnies."
'Media circus'
The next day, Tory leader Edward Heath sacked Powell from his shadow cabinet as the debate over his speech and arguments over immigration reached boiling point.
The country demanded to know who this woman was but Powell repeatedly refused to say.
The whole of Wolverhampton was trying to find out where this street was and who this woman was.
The BBC's former political correspondent Nick Jones, whose family were close friends with the Powells, remembers the media circus that followed as reporters tried to track down the woman.
"The whole of Wolverhampton was trying to find out where this street was and who this woman was. The Express and Star failed to find the woman or the street, the Sunday Times failed to find the woman or the street," he said.
Archive trawl
To this day nobody has been able to answer either question or prove allegations that Powell had made the woman up.
With the help of historian Dr Simon Burgess, Document travelled to Wolverhampton and trawled through electoral records, old phone books, newspaper cuttings and boxes of city archives, in an effort to solve the mystery.
Mr Powell said the woman lived in his Wolverhampton constituency
Finally, after extensive research, Dr Burgess came up with the street where he believes the woman Powell quoted in his speech lived.
Brighton Place is in the MP's old constituency of Wolverhampton South and isn't far from his former semi-detached home.
By 1968 it was almost entirely populated by immigrant families, except for a 61-year-old white woman living at number 4.
Immigration increased
Dr Burgess said: "I've been boiling down the 200 names that we arrived at and managed to find one individual who matches most of the essential points in the letter. And I can actually put a name to the face by saying that she was Drucilla Cotterill."
Just like the pensioner Powell quoted, Drucilla Cotterill owned her own home, lost her husband in the Second World War and stopped letting out her rooms to lodgers when immigration increased.
Other former residents of the street, which is now Brighton Mews, have confirmed to Document that excrement was pushed through a letterbox in this street and that nearly all those living here were black in the late 1960s.
So, given that Drucilla is the only person who fits the profile of Powell's pensioner, has this mystery finally been nailed?
Lord Howard of Rising seems to be in the unique position to answer that. He is currently the shadow minister for Culture, Media and Sport but was Powell's private secretary in 1968.
Enduring mystery
Before my arrival at his home, Lord Howard confirmed that he knows the name of the mystery woman. So, after all this time, is he prepared to give it to me?
"No," said Lord Howard.
"He (Powell) had a very strong view that if anybody who wrote to him in confidence, that should not be broken."
We've come up with a name and that name is, Drucilla Cotterill. Does that ring any bells with you?
He replied: "Listen, I'm very pleased that you've done all that work. I'm sure if you've done it, it's very valuable but don't expect me to comment on whether it's right or wrong because I've already told you I'm not going to say."
So, when do you think it'll be that the public will eventually find out for sure the identity of this particular lady?
"When I'm six foot under," he said.
The mystery looks set to endure just a little while longer.
Document is broadcast on BBC Radio 4 at 8pm on Monday 22 January.
