You'll always find a bit of grass somewhere, unless you want to sleep right next to you bike?
Why not get 2-3 to chip in £75 each and buy a heeeeuge party tent type thingy like what I have.
6m x 4m like a double garage, was about £180-£200 I think maybe less. Have a look on ebay.
You can then all park your bikes in that securely overnight, and camp wherever you want.