Captain Mike Yates during the Pertwee era of Doctor Who was played by Richard Franklin. Franklin made his last regular appearance as Yates in the Pertwee swansong Planet of the Spiders although he made a cameo in the twentieth anniversary The Five Doctors but he turned out to be a phantom and not the real Yates.
So Planet of the Spiders was the last time that the real Mike Yates was seen in the Doctor Who TV series.
Franklin in the 1990s decided to write what became of Yates after the events of Planet of the Spiders in a novel called The Killing Stone but it was unpublished. The Killing Stone however was adapted as an audiobook released by BBV in 2002 and read by Franklin.
The Killing Stone plays like a Doctor Who story only it mostly focuses on Mike and this came years before the Doctor-lite episodes in Doctor Who.
As well as Yates there are appearances substantial or fleetingly by the Brigadier, Benton, Sarah Jane and even the Master and the Fourth Doctor.
Franklin has written a very good story about Mike Yates that is ultimately about Yates’ redemption due to the circumstances that he had left UNIT in Doctor Who.