This was nearly Target's 150th Dr Who novelisation

Back in 1990, Target Books was fast running out of classic Doctor Who television tales to publish as novelisations. So the editorial team got permission to adapt lost stories from a cancelled series that would have starred the sixth Doctor and Peri, including Mission to Magnus by Philip Martin. Indeed, Mission to Magnus was planned as Target's 150th Doctor Who book, as is proven by this printed cover I was given while visiting the editorial office in 1990.

It was eventually decided to issue the books as their own series, The Missing Episodes, rather than driving a generation of fans crazy by incorporating the non-broadcast stories into the numbering of 'canonical' stories.