AFTER 26 years, more than 4,500 shows and countless vowels and consonants, Carol Vorderman is finally hanging up her chunky black marker and leaving Countdown. The 47-year-old Cambridge graduate, who has been the show's number queen since it first aired in 1982, makes her final appearance on Channel 4 this afternoon. She will be replaced by 22-year-old Rachel Riley, who fought off competition from more than 1,000 others to take over from Vorderman for the new series in January. Today's show also marks the end of Des O'Connor's time in the host's chair - he is to be replaced by Sky Sports presenter Jeff Stelling. O'Connor took over the words and numbers quiz in Jan 2007, replacing Des Lynam, who stepped in following the death of much-loved Richard Whiteley. " AN