I just got home to find the above. I am devestated, she was the greatest companion ever.
For those who don't know, Liz Sladen played Sarah Jane Smith, who played Doctor Whos' companion in the 70s (Jon Pertwee/Tom Baker era). She has been consistently voted as the greatest companion ever and had gained new fans in The Sarah Jane Adventures which had been showing on CBBC for the last few years.
I am off to find a Who dvd now to remember :(