In January 2010, Marshall co-ordinated the 10:23 Campaign with Mike Hall and Andy Wilson to stage a mass overdose of homoeopathy outside branches of Boots UK in several major cities throughout the country, to publicly demonstrate the inefficacy of homoeopathic products and protest against their sale.

In November 2019, Marshall was in Brazil to officially launch the campaign that aims to withdraw financial aid for homeopathy by the country's public health system (SUS).In October 2011, the Merseyside Skeptics Society (represented by Hall and Marshall), Chris French and Simon Singh set up a "Halloween Challenge" to Sally Morgan to have her alleged psychic abilities tested, to demonstrate that her claims regarding talking to the dead are true, otherwise she might, knowingly or unknowingly, be taking advantage of people's grief.

[22] Episodes of the show include interviews with Flat Earther Mark Sargeant; Vicki Monroe, a Psychic and cold case investigator; and Jim Humble, a proponent of Miracle Mineral Supplement solution.

Such press releases are more often than not simply veiled advertisements, disguised as scientific studies or representative social surveys which, if based on any sort of inquiry at all, are usually poorly set up or conducted and prone to bias.

Moreover, the headlines under which these "results" get published can be sensationalised and thus even more misleading, and if readers believe such "news" stories to be true, it may have serious negative effects on people's views and actions.

[26] When education secretary Michael Gove was criticised by many mainstream newspapers for mistaking a PR stunt by OnePoll for hotel chain Premier Inn, for genuine research on schoolchildren's allegedly lamentable knowledge of British history, Marshall called this "ironic", and rebuked the newspapers by showing how they themselves are largely relying on the same kind of agencies' press releases with "dodgy surveys" for their news stories.

Namely, as Marshall states, "it is striking how many people who doubt the global model of the Earth also subscribe to all manner of other beliefs, from Biblical literalism to occultist paranoia, from anti-vaccination to quack cancer cures, from antisemitism to Aryanism.

[33] His major focus has been ending the funding of homoeopathy by the National Health Service (NHS), which the GTS considers a costly waste of public money on demonstrably ineffective products;[34] he lectured about this at QED 2015 in Manchester.

"[36] Also in June, he and the GTS founder Simon Singh called on all remaining homoeopathy-funding CCGs in the UK to follow the example of Liverpool to reconsider their funding policies.

Recalling the British Chiropractic Association v Singh case, he concluded, "If chiropractors want to be taken seriously, perhaps they should focus on improving the regulation of their industry and conducting rigorous research rather than relying on PR stunts to drum up business.

"[39] In September 2015, Marshall showed how the American televangelist, self-proclaimed prophet and faith healer Peter Popoff – previously exposed by James Randi– was trying to persuade people to send him money on promises of "fabulous extreme fortune" and "miracles".

In June 2018, BHA lost that case, in a decision characterised by Edzard Ernst as the result of "4 years of excellent work by the Good Thinking Society".