Although he had also previously adapted stories for Agatha Christie's Poirot, in 2002 he made a further move into writing "straight" drama, with the fantasy horror series Strange.
Around the same time, he worked regularly on Radio 4's Week Ending, together with David Renwick, Douglas Adams, Alistair Beaton, John Lloyd, Simon Brett and others.
Brought by Humphrey Barclay to London Weekend Television, originally to repeat his 'nursery slopes comedy' Do Not Adjust Your Set with End of Part One, Marshall and Renwick went on to write a series of television satires, including Whoops Apocalypse, Hot Metal and If You See God, Tell Him — the latter originally for Channel 4, but postponed for several years when the channel refused to let them direct it, and finally ending up at the BBC later.
After a tentative and unsuccessful attempt at solo writing with Sob Sisters at Central Television, Marshall found long-lasting success BBC One's 2point4 children, adding to it Health and Efficiency and later, Dad.
He later began a new collaboration with Rob Grant, producing directing and writing the Radio 4 Series "The Quanderhorn Xperimentations" - and also the novel version published by Gollancz.