From his rather mysterious Wikipedia entry:
In 1985 the fourth title in the Horace series, Horace To The Rescue was announced. though this game never appeared. During the development of the game, Tang suffered a collapsed lung. There is no more current evidence of his involvement in any other titles after Asterix and the Magic Cauldron or any other information post 1985.There are a number of William Tang's in Australia listed on LinkedIn, but none seem old enough to be him.
