I had another look; a lot of the material easily available seemed to be on newspaper websites. But then I found this researcher: Dr Rachel Pechey
It was her article that drew my attention to her work:
Pechey, R., & Monsivais, P. (2015). Supermarket choice, shopping behavior, socioeconomic status, and food purchases. American journal of preventive medicine, 49(6), 868-877.