You need to remove the four hex screws with an suitable sized allen key, then use those radio removal keys, (you can get them from any good motor shop reasonably cheap - or ebay).
I recently removed my partner's head unit without the aid of the special keys using a wire coat hanger, (cut up into strips of bendable metal).
Insert the special keys, (or pieces of metal) into the four holes where the grub screws came from and with all four inserted should push back the metal clips holding the cage in. From here the head unit should slide straight out, (I found some coaxing from a flat headed screwdriver either side of the front helps it along abit too.
Hope that helps.
Matt
March 2010