Ok so this is a little embarrasing/confusing.
It would seem that I did not copy the correct public key onto my repo management web interface. The thing is, I copied the public key from git bash using the command:
clip < ~/.ssh/id_rsa.pub
But that seems to give me a different public key from what I actually get opening id_rsa.pub in a text editor...
Is that normal? Why is it different? Why does it work within gitbash and not in SourceTree (via pageant).
Anyway, copying the contents of my public key in directly from the file when opened in notepad got things to work with SourceTree and Pageant etc.