Having decided not to fit the carpets underneath the seats, I debated the need to fix the mats to the floor pan. They are rubber and seemed to have a high friction when placed on the floor. The thing that changed my mind was the thought of the driver side one moving over time and becoming rucked under the pedals.
The kit is supplied with press fit stud fasteners that are riveted through the floor pan and punched through the floor mat. I decided to fit three across the width of the rear of the mat. The location of the studs was marked on the rear of the mat, and a hole punch was used. The mat was put into position on the floor pan and a chalk marker was used to transfer the location of the holes onto the base. Holes were drilled in preparation for the rivets and the studs put into position. The rivet gun was unable to reach fully into the stud and so a suitably sized nyloc nut was used as an infill.
Floor mat marked with stud locations; fitting button studs; using a nyloc as an infill for the rivet gun