Leo loves cheek and chin skritches. Even chest rubs if he's particularly amenable (not often).
Simon likes his cheeks and chest scratched too. If I start scratching under his chin, he'll stick his head in the air and stretch his neck as far as it'll go and grumble at me. He looks like a plucked chicken when he does it.
Can't scratch their backs tho. Simon won't allow it and Leo will either immediately turn around and start chewing his back, or chewing your finger, or chewing whatever is closest to him whether it be a carpet, a carboard tube, anything. He chews HARD.