What an ordeal Annie - sounds like you had an episode or rogue traders and casualty rolled into one...well done for sticking to your guns and I'm glad your boy is now well.
we used them in our field gateway! someone who used to have an electric car on these grass mats got fed up of lifting them out of the grass and gave them to us!
We had previously put down a lot of hardcore that had eventually sunk and been covered in mud and it has made a good base for the mats to go on! there was sufficient mud/ soil between the mat and the hardcore for the grass to grow through. It's done quite a good job, you can hardly see the mats are there in places in the summer.
If you wanted it for actually grazing, it's not so good, the grass seems to grow in 'plugs' and the horses can pull those out! Xx