All of the ones I've just checked seem to be closed. (Those at railway stations and filling station seem to be trading normally though). Some of the town centre ones might be big enough to fall under the 'large stores' rules, meaning that they're prohibited from opening on Easter Sunday (unless they're in Scotland, of course).
Start here:
http://www.marksandspencer.com/
Click 'Find a Store'
Enter town name and click 'Search'.
Click on a store, then on 'Opening Hours'. (Where there are special hours over Easter, they should be shown there. If no alterations are shown it suggests that there will be normal trading).