OK, so I tried the bathtub cure, but the mat is quite large (markings are 28 x 56") and it was a pain to keep turning it in the bath.
I had more luck sandwiching the mat in an electric blanket and leaving that on overnight. It still warped very slightly as it cooled, so I will repeat the procedure tonight and put the old mat on the new one and weight that all down as it cools tomorrow.
Again, these are great mats, I am very fond of mine!