I wanted to have him there purely because if I made stupid mistakes which meant I failed, I knew I would instantly forget what I did exactly and wanted him there to help me to learn from them. Anyway in the test he sat behind me so I could barely see him anyway, he clearly tried to stay out of the way when I was reversing or looking out of the back window and didn't distract me in anyway, so I wouldn't say it was a bad thing at all, anyway I passed first time so it didn't matter in the end. Also he was really nice afterwards, saying that he was willing me to pass the whole way round and after about 10 minutes thought I would pass, which made me feel better as I still think it went pretty badly.
But afterwards it made me feel more reassured as he just kind of chatted with the examiner while he made his final notes I panicked in the drivers seat, knowing I couldn't talk at that point. Then again a friend of mine took 4 tests, the first 3 she didn't have hers with her, on her last one after what I said she decided to have him with her, but her examiner was being moderated (by someone who had done one of her previous test), so there was already an additional passenger so she decided to go without him and passed anyway. So clearly having two people in the car (and feeling judged by them as well) didn't affect her.