Observation study permissions required?

Hi

I'm in my final year of PGDip psychology and my final year project requires me to go into an NHS hospital and sit in the canteen to monitor where staff sit. My supervisor has tentatively said she doesn't think permission is required from the NHS as the area is a public domain. I'm a bit weary of this approach, as the canteen is public domain I am guessing some people may be patients. Although my obs study is only interested in staff, I'm just wondering if my turning up each day for a week, possibly for an hour or two at lunch time, to record seating plans requires NHS permission?

I will fill in my ethics form before collecting data, but just wanted to query to see if anyone here had any ideas? I know the ethics board will say yay or nay but I just wanted an idea now, and not in a week after the ethics form has been reviewed.

Any feedback will be appreciated!! Thank you