I think the quality has really done down. Not enough lycra or stretchy stuff - they end up mishapen and ride up in all the wrong places. They must have changed the formula because I have been buying their 'briefs' multipacks for years.
I was unfaithful and tried a John Lewis multipack. So, so, so much better. Thicker,more lycra and wash really well. Similar price.
Sorry M and S, I think you pants are pants.