In the radio series Beyond Our Ken, she played Fanny Haddock, a takeoff of Fanny Cradock. In the radio series Round the Horne, she played a similar role (Daphne Whitethigh), as well as Lady Counterblast (née Clissold), Buttercup Gruntfuttock (wife of J. Peasemold Gruntfuttock, personified by Kenneth Williams), Dame Celia Molestrangler, Judy Coolibar, Dame Bella Goatcabin, and others.
Perhaps her most famous catchphrase was "many, many, many times", delivered in the dry, reedy tones of Bea Clissold, the ancient actress who was renowned for having given pleasure to many, particularly in "The Little Hut" on Shaftsbury Avenue. This long outlasted the Clissold character and was deployed to much audience appreciation on a few occasions in later series, possibly as an ad lib. Another was "'allo, cheeky face!", shouted into the microphone in the less-than-couth London tones of Buttercup Gruntfuttock. Marsden's vocal range was impressive and also included the husky Whitethigh, the strident stereotypical Aussie tones of the ultra feminist (but conflicted) Judy Coolibar, and the cut-glass received pronunciation of Dame Celia Molestrangler (in a series of loose pastiches of the stilted dialogue in 1930s' melodramas).
It has been reported that, during the filming of Carry On Camping, Marsden told fellow actress Dilys Laye that she wanted to die with a glass of gin in her hand and that in July 1998, merely a day after moving into a residential home for old actors, this is exactly what happened. Reportedly, Marsden had been chatting to friends in the home's bar when she collapsed into a chair, with such apparent grace that she spilt not one drop of her drink.