So as I understand it, you can:
A) Have the 6006 taken out. Buy a single din fascia adaptor from Ford for about £25-30 (can get a whole kit cheaper online at the
Nexxia site for example). Get an audio installation company, or DIY, cut the two thin strips of plastic & fit the fascia adaptor (like the pics in Chris' post). Fit the new headunit & have a handy wallet/phone/sunglasses/sweets/iPod/whatever hole underneath *heh*
B) Have the 6006 taken out. Buy a sat nav fascia (bascially a replacement for the whole part of the console where the stereo sits without having to cut the plastic strips) for about £80. Fit new headunit etc etc etc...
Like Parksey says it depends if you want to sell the car on afterwards or how much you want to spend. I trust the install company I'm using to cut the dash and seeing as I'm planning to keep this car for the forseeable future, I'm not fussed about cutting bits.