Grahams
Don't complain - suggest what's better
Obviosly in the picture you are using the ply in a different context, stopping steel tracks marking a hard surface which ply is ideal for. There is a danger to the steel ripping the grippers in your situation on poly.18mm ply, lasts 2-3 years providing you store it upright with batton between sheets to allow it to dry. I've taken the 7t across lawns before no issues with cracking provided you use your brain when placing it out.
Graham