I then ran WinDirStat and found that the winsxs (C:\Windows\winsxs) folder was very large.
After a bit of research I managed to use the command below and reduced the winsxs by almost half.
DISM /online /Cleanup-Image /SpSuperseded
The command needs to be run from an elevated (i.e., Run As Administrator) command prompt, and you cannot uninstall the Service Pack after it completes.
I then did the same on my other Windows 7 laptops and recovered an almost identical amount of space.