The seat webbing can be replaced, it's not a fantastically difficult job, but you do have to remove the seat cover. About ten years ago my seat webbing split just before I was going to Europe on a longish drive. I didn't have time to fit a new piece of webbing so I grabbed a spare roll of strimmer line that I had kicking about and just laced back and forth across the seat rails. It worked fine, so much so that it's still there.
