As a hairdresser myself we have just got this new product called 'Urchin Exit' which breaks up the colour molecules in the hair without causing any damage at all, in fact it actually deep conditions your hair too, leaving it soft, shiny and strong. Ring a few salons to see who stocks it...it's an Australian product but I'm almost positive they have it in UK salons. They apply it to your dry hair just like a colour and after 20 minutes or so the formula is washed off, taking out all the semi-permanent/permanent colour. Exit will not remove highlights as the natural hair has had its colour stripped out and cannot return to it's natural colour, so any bleach mishaps really need to be covered with a darker permanent dye.
Anyway, I hope you find it...it shouldn't cost more than having a colour done and you can have your lovely locks back in less than an hour without having to damage it further by over washing! :)