I am thinking of applying for a PhD, but since my degree is from the Open University there is not the same tutor contact; so who would I use as referees?

I would still put someone from the OU as your referee if possible. Most lecturers don't know their undergraduate students very well so they are used to producing references based almost wholly on academic records. Good luck.

Yes - try to use two academic referees. You can use a third referee if you're worried that the academic ones don't actually know you.