Wallis has reported on the British Post Office scandal since 2011, having become aware of it while presenting on BBC Radio Surrey.

[5][6][7] From 2018, having left the BBC, he started reporting the Bates & Others v Post Office Ltd trial on a dedicated blog, www.postofficetrial.com, having raised £9,000 through crowd-funding.

[3][8] He co-wrote Private Eye's 2020 report "Justice Lost in the Post".

[3] He acted as a consultant on the 2024 television series, Mr Bates vs The Post Office.

His book, Depp v Heard: The Unreal Story,[11] about the trials in both the UK and the US and his experiences reporting from court was published in May 2023.