It looks like Marks have changed their sizing. I have bought regular length for many years now but of late the trousers arrived so long I had to take them all up which was a proper nuisance as I like to do it properly with a sewing machine and cut the excess fabric off as it ruins the "hang" otherwise.
I went to buy these trousers online today and the regular is 31 and the short/petite is 29 inches.
Im 30 inches so it looks like I'll be hem bothering forever as the 29 inches are ankle flappers on me and the 31 inch trail on the floor.
www.marksandspencer.com/cotton-rich-retro-side-stripe-joggers/p/clp60726382?color=ECRU#intid=pid_pg1pip48g2r1c1