I've been dyeing my hair black for about 15 years but recently decided I wanted to go red.I bought Scott Cornwall colour remover(About £12) from Boots,I used the extra strength and bought 2 packets as I have long hair.It removed a lot of the black with the first pack but I put the 2nd pack on for good measure and it worked wonders!!My hair went back to a shade very close to my natural colour.You MUST on the final rinse, rinse for at least 10 minutes and as much as you get fed up with hanging your head over the bath for that long carry on with it.A friend of mine used it and got bored of rinsing so gave up after a few minutes and as her hair dried you could see the hair getting darker and going back to the colour she was trying to get rid of.
HTH