I'm an idiot! I completely forgot I had installed Forza Horizon 3 which doesn't show up at all and that's about 60GB (LOL). It comes under 'Windows Apps' which is hidden in This PC but shows when using WinDirStat. Thanks guys!
I have a 6GB hibernation file and a 15GB Windows.old folder (which is apparently from upgrading from an older OS) but this was a fresh install of Windows 10.
Should I delete the hibernation file and the Windows.old folder?