If you look on Trustpilot, M&S has dreadful reviews, particularly since the standard of clothing went down, cheap material, poorly sized and poorly made, in places like Bangladesh, which screams sweatshop workers. Their customer care is now outsourced to South Africa, and is definitely not what M&S customers have been used to over the years.
I used to always buy knickers from them, but over the last few years the quality has markedly declined, and the sizing is ad hoc to say the least. Shame to see this happening to so many British companies when they were really good for so long.