Malone
Congratulations on your apostrophe-free posting, difficult when writing about these companies. It reminds me about the story that was written without using the letter "e".
Sainsburys (as we call the company) brand themselves in 4 different ways, Sainsbury's/Sainsburys/Sainsbury/ J Sainsbury when referring to their stores/website/on-line shopping/stock market.
None of these is correct. It should be Sainsburys' as it was a family business run by several members called Sainsbury, who I think still run it!