This news for season two of the Sarah Jane Adventures is from The Doctor Who News page over at GallifreyOne.com:
The returning character is Sir Alistair Gordon Lethbridge-Stewart, played by Nicholas Courtney. The character - who for much of the classic series was the commanding officer of the UK branch of UNIT, and one of the Doctor's closest allies - first appeared in 1968 opposite the Second Doctor, and went on to appear in stories with the Third, Fourth, Fifth and Seventh.
Courtney has also played the Brigadier opposite the Sixth and Eighth Doctors in officially-licensed audio dramas from Big Finish Productions.
The same Sarah Jane Adventures story will also apparently see the return of the Sontarans, seen in several classic series stories and more recently in series four of Doctor Who. Samantha Bond is also believed to be returning as Mrs Wormwood, the character she played in the pilot episode of The Sarah Jane Adventures.