Linda Shaw, 37, said she recognised the feet as Miss Allan's, and what she saw of the man's naked buttocks appeared to resemble Mr Terre-Blanche.
She said she had discussed sex and Mr Terre-Blanche with Miss Allan. 'She said he was a great lay but a little heavy,' Miss Shaw added. Miss Allan, 40, who now lives at Hampton Court, Surrey, is suing Channel 4 Television over the film The Leader, His Driver and the Driver's Wife, which she claims portrayed her as a 'lady of easy virtue' who slept with Mr Terre-Blanche.
Channel 4 says it never suggested an affair, but argues that such an allegation, although never made, would be justified because Miss Allan did have an affair with Mr Terre-Blanche, who is married with a daughter.
Miss Shaw, said she first met Miss Allan in 1981. Miss Allan had confided in her about her crumbling marriage, which ended after two years in 1984. While Miss Allan was still living with her husband, the two women used to go out to restaurants and bars and often came home late, sometimes not until 4am.
She said Miss Allan had made it fairly clear to her that she would not be happy to live under a black government in South Africa and would sometimes call black people kaffirs.
Miss Shaw told the court of accompanying Miss Allan to a rally of the neo-Nazi AWB party, when Mr Terre-Blanche had appeared to be addressing the whole meeting to Miss Allan, who was sitting in the front row.
One night she had woken at 3am to find a large man in khaki in their flat. He said that he had been sent up by Miss Allan to have sexual intercourse with her because she liked sex very much. She screamed, and got rid of the man. Later she heard stamping of feet passing her bedroom, followed by an 'incredible noise' which sounded as if all the electrical appliances in Miss Allen's room had been turned on. Miss Shaw said: 'I was absolutely petrified. I thought they were maybe hurting Jani and using the noise to cover up the sound of her screams. I crept along and tried calling her name. There was no response. I tried knocking. There was no response. I turned the handle and it was locked. So I looked through the keyhole.' She could see Miss Allan's naked feet flat on the floor with her knees up and apart. She could recognise the feet because her flatmate was neurotic about them and was always asking for reassurance that they were not ugly.
George Carman QC, for Channel 4, asked her: 'You said you saw a bottom. What colour?'
Miss Shaw replied: 'White. It was a large bottom on top of Jani, rising up and down. I assumed they were having sex.'
She said she had never seen Mr Terre-Blanche naked, but from what she had seen of him clothed she thought the bottom she saw was the right size and shape. She had seen the toes of boots on either side, which she took to be those of bodyguards.
When she spoke to her flatmate the following day, Miss Allan had said: 'Sweetie, you've had a nightmare.'
About this time, Miss Shaw said, 'Jani thought she would be the new first lady of South Africa when Terre-Blanche took over as president or whatever. She was buying clothes she thought would be appropriate to that. Very classical Italian clothes.'
On Miss Allan's birthday in September 1988, the two women had talked for hours and drank 'copious' amounts of alcohol. At midnight, after receiving a phone call, Miss Allan drove them in her red Lancia to Krugersdorp where the met Mr Terre-Blanche, who was also very drunk.
After half an hour embracing and cuddling Mr Terre-Blanche, Miss Allan drove them all back to Johannesburg. When they arrived at about 4am, Miss Allan fell over and could not get up again, the court was told.
Miss Shaw said that in November 1988, Miss Allan had told her the AWB planned to kill both women because they knew about the affair with Mr Terre- Blanche, and it could be damaging to the movement.
She decided to leave the country when some dead white roses were sent to Miss Allan's flat.
Miss Shaw said she was in Hong Kong visiting her family when she saw a television news item about the incident at the Paardekraal monument in December 1988, when Miss Allan was found with Mr Terre- Blanche. When she returned to South Africa in mid-January 1989, Miss Allan would not open the door to her or return her telephone messages.
The case continues today.