The pair fell in love after playing Lancelot and Guinevere in the 1967 musical Camelot and had a son, Carlo, who is now 40.
They split shortly after he was born but Redgrave insists she never felt she and Nero had parted - and they decided to declare their love for one another before family and friends in 2006.
The 78-year-old actress admits she snubbed a legally binding ceremony because she didn't want to deal with "all of the money business" official unions require.
And Redgrave, who plays Nero's reunited lover in new movie Letters to Juliet, insists she feels "terribly lucky" to have enjoyed a lasting romance.
She says, "It was quite clear to us all that what we didn't want was a marriage which was legal and which has money involved. Stay clear of all that, stay clear of all the money business. Stay clear of being legally married.
"We were married in front of family and friends with the greatest possible commitment and love. I think I'm terribly lucky, terribly lucky.
"We had never really separated. We had plenty of times when we weren't speaking to each other or when we were shouting at each other. Mostly the worst times were when we were not speaking to each other."