'Spiral Scratch', written by Gary Russell, features the Sixth Doctor and Mel.
Back Cover Blurb
Carsu: the largest repository of knowledge in the universe - in any universe, for there is an infinite number of potential universes; or rather, there should be. So why are there now just 117,863? And why, every so often, does another one just wink out of existence?
The Doctor and Mel arrive on Carsus to see the Doctor's old friend Professor Rummas - but he has been murdered. Can they solve the mystery of a contracting mutiverse, and expose the murderer?
With the ties that bind the Lamprey family to the past, present and future unravelling around him, only the Doctor can stop the descent into temporal chaos. But he is lost on Janus 8. And Schyllus. And a twentieth-century Earth where Rome never fell. Andů