I want to be able to access alternatives to Unity as it really slows things down with only 2GB of memory.
I thought that if I could get unity-greeter to run that I'd have a choice.
You probably have "Automatic Login" switched to On. You need to switch this off.
Go To System Settings > User Accounts > [Select unlock]
Then use the toogle switch to turn off "Automatic Logon"
Then Log out and back in again and the Unity Greeter should be there.
You will be able to select options other than Unity here,