I guess really you could say the whole thing sucks.
Useless fact: You can have any size extractor fan in a bathroom, but it will only be as good as the air that can get in to replace it. Ideally there should be 1/2 inch gap under the door. (But it is rarely mentioned)
I fitted my own complete MVHR system when building my new self build house a couple of years ago. I was able to borrow the anemometer kit to set up all the flow rates, but having done that for my own peace of mind, building control never asked for any paperwork.