Open Question: Convincing my doctor I have depression (UK)?

I want to see a doctor for my depression today. I am very worried that he wont believe me, however, or he would think I only have a slight depression. I have several years of suffering and have only just won the confidence in the pick up to help – I really do not want him disappointed that he did not ignore me. I do not know what to say? Or will he ask me / to me? .