Douglas was the script editor for one season.

The Doctor dismisses a book by Oolon Colluphid in Destiny Of The Daleks, it's largely accepted as Fanon that H2G2 takes place in the same universe as Doctor Who just as Quatermass does (the Doctor claims to have met Arthur Dent in The Christmas Invasion which also mentions the British Rocket Group).

Life, the Universe and Everything was largely based on the abandoned Doctor Who script Doctor Who and the Krikketmen.

Dirk Gently's Holistic Detective Agency has whole sections lifted from the Doctor Who stories Shada and The City Of Death both written (mostly) by Adams under assumed names.

Originally the Daleks were to turn up at the end of season two of Blake's 7 some of the spin off material accepts that join too and have merged the two universes.