I've heard hair dye removers mentioned a couple of times in threads and wanted to remove the build up from dying my hair dark red/browns. Couldnt find the one in boots but managed to get a cheap one from savers 4.99 and i'm really pleased with the results, its not a quick process, be prepared to spend an hour and you may want to have a hair dye ready for afterwards - it talks about how even though you have dyed your hair darker some dyes actually lighten the hair first, so my real mid brown colour was a bit gingery afterwards, so i dyed it a mid brown the next day.
I'm glad i found this product so now my hair doesnt have to get darker and darker each time.