Just be aware that it is actually illegal to just pitch anywhere your hat falls.
However, i come from Redcar originally and i think the Eston Hills have some nice spots. I couldn't tell you where but i had friends who used to disappear for a night or two up there and driving past them everyday to college and work, it does look interesting.
Also not far past Thirsk on the A170, it takes you into The North York Moors National Park. Obviously camping at your own risk.
I know its not much help, but its a start i guess.
Stacey