Here are some links to interesting stuff about Felix. Please cut and paste the URL into your browser if you’re interested. Simon.

An interview with Felix for the American Institute of Physics oral history project, by David Rickles, in 2011:
https://www.aip.org/history-programs/niels-bohr-library/oral-histories/34463

An article about the centenary of the theory of relativity, mentioning Felix’s correspondence with Albert Einstein:
http://www.nytimes.com/2015/11/24/science/a-century-ago-einsteins-theory-of-relativity-changed-everything.html?_r=1

A photo catalogue of Felix's mosaics:
https://www.flickr.com/photos/69491183@N06/albums

A wikipedia entry (which has more links):
https://en.wikipedia.org/wiki/Felix_Pirani

Relativity at King’s College, from the college web site:
http://www.kcl.ac.uk/nms/depts/mathematics/about/relativity.aspx

An interview with Felix in the New Scientist, February 1971:
https://piraniarchive.files.wordpress.com/2016/01/activism-with-feeling.pdf