My partner and I are purchasing a property together. I am in full-time employment, he is re-training and has just gone self-employed and is currently not utilising his single person's tax allowance. However, I am paying the 25% deposit. How should we divide the ownership of the property to maximise any tax relief?

From the wording of your question "My partner and I" I get the impression that you are not married. If so, you can divide up the actual ownership of the property in whichever proportion you want, and then make a written agreement between you (preferably before you start renting out the property) that the rental income will be received in a different proportion - one that utilises your partners' personal allowance and the basic rate band of both of you in the most efficient way. Then make sure that the rental income actually goes to your individual bank accounts according to the pre-agreed amounts