Also forgot to mention, there's a physical resale booth within the grounds where you can put your name down for returns on the other show courts (not the one you already have tickets for). There's a virtual queue and you normally get a notification after about 6pm that seats are available, and you can go and purchase them. From memory they're around £20. They can be anywhere in the stadium, but it's often the posh seats where people leave early.
This isn't really a practical option in the second week of the tournament though, as there are fewer matches and people mostly don't leave early.