Just thought I would update this, but we did get a catmint plant for His Highness, and it's gone down a treat!
He hasn't destroyed it, yet, but every time he is let out he heads to his little corner of the patio, which has the catmint in a pot, and also a pot with cat grass in. He loves rubbing on the catmint, especially if we go over and squish the leaves a bit, then he will have a good nibble! Then he will nibble the cat grass, and head off for his adventures
So in summary, a success!