I'm an engineer and I wouldn't just look for "a mechanical engineer". You want someone that has a good understanding of fan curves, air changes per hour, possibly the clean air act, etc.
We've got a consultant whose name I could give you, but that would probably work out a bit more expensive than going through a chartered engineer.
Edit: I had my window open before going out and didn't read the other replies. Seems like my suggestion would probably be an overkill.