Never mind that, Windows 10 did an update a couple of days ago, and now my dual boot screen has gone. I literally can't start my Linux Mint anymore, it boots straight to Windows :(
There is a way to get it back with the command line, but my computer nerd days are over.
https://askubuntu.com/questions/911963/dual-boot-with-windows-10-broken-after-update
I'm assuming that command needs to be edited to whatever my setup is, I have no idea.
My Linux is on a separate hard drive.