Hi Barry, I had the same problem in the top left corner so that the damaged Germans box was crumpled.
All I did was roll the mat the other way i.e. against the direction of the creases and then put a couple of large elastic bands around it and left it for a week; the creases pretty much ironed themselves out in that time but also leaving the whole thing flat on the dining room table after the week was up helped flatten it too. Admittedly I do still have some discolouration where the creases were but it's no biggy.
Whatever you do, don't iron it!!!!
You could use a hot air dryer to heat it up and work the crease out though.