I read about this yesterday. It was one of those real gutpunch passings that just comes out of nowhere and leaves you kinda speechless, made doubly sad by the fact that I was just starting to get into some of the older Who and really enjoyed the episodes with Sladen as the Doctor's companion.
I wonder if they'll show the episodes of Sarah Jane Adventures that she did manage to film before she passed. IIRC she was half-way through the season. maybe a day-long marathon as some sort of tribute?