Ok, so it's not going to change anyone's life, but I thought someone else might find it useful to know.
I regularly dye a batch of clothes black as we seem to wear a fair bit of it and it fades (even though I use colour washing liquid stuff). I spotted this in Sainsburys, you just bung in a sachet in the machine with your black stuff run a wash and whoopie all back to black. Two sachets in a pack, can't remember how much it was.
No, I don't work for Dylon.