As a medical man myself, I can see this app becoming a royal pain in the ass for me. The problem with self diagnosis apps is that patients tend to treat multiple choice options like dishes in a restuarant:
"Let's see, have I got a headache....well, it feels a bit funny, so I'll say yes and my stomach feels queasy that had absolutely nothing to do with the raw 3 o'clock kebab last night, so that's another yes.....oooo, looks like I have bubonic plague, better go see the doctor..."
"No sir, you haven't got plague, you're just hungover."
"Worthless quack, this NHS DIrect app says I have plague."
"I see. Did the programmers do ten years of medical school?"
"No but it does have a very shiny interface."