I believe it is highly recommended to have a Karndean fitter. However this might be just some hype to get you to part with more money!. I think they charge about the same as the price per sq mtr. eg. £25 per sq mtr becomes £50 per sq mtr!!!!!!
A word of warning; a friend who has it has to 'strip' it every 6 months and reseal it!! Couldn't be doing with it!
My son had some laid in his family room by a karndene fitter and it looks great but when he had his hallway done the local carpet shop said they could fit it. They did but had to come back 4 times to correct it where it had lifted or shrunk back. Maybe just bad luck but the same fitter made a fab job of the staircarpet so maybe Karndene is difficult to fit? I would also agree it needs to be stripped and repolished every 6 months but is good wearing