I cloned the cement bench, because I'm busy making a few benches, some without backs, but I've found that the sims still treat the bench as if it has a back. The bench only has a single bone in the mesh and I'm not sure that it depends on that bone.

Is there somewhere else I need to look to fix this or should I basically live with it?

Ps. I know the texture is off

Changing the mesh won't change how the sim sits. You could change it to a pitchfork and sims would sit on it happily.

Thanks cmomoney, this I knew I was more wondering about the rigging, but since there is only one bone I'm guessing I'm stuck with how this looks, unless I clone something else which gets them in a more upright sitting position.

The rigging only says where the sims sits. How the sims sits is baked into the animation. You could, as you said, clone something closer to the animation you want.