I woudnt even bother, if its mouldy then your best off getting a CMEV fan "Continuous" fan.
I hate ventilation but i know abit about it, without getting overly technical.
For a normal intermittent (over-run/ humidistat) fan to comply with Part F you need alot of background ventilation, (fine in a single glazed house) when you do the math a standard 3 bedroom house with trickle vents on every window most likley wont comply as you physically cant put enough windows in a property to get enough background ventilation.
Unfortunately nobody seems to know this. New houses are made and no trickle vents installed, flexible ducting everywhere, no undercuts under doors.
Then they wonder why the house stinks and they get moyld up the walls.
Good rule of thumb is as Ruston said, look through ever room check you have 9mm undercuts under each door, then if i were you id put in continuous extract van and be done with it.
I use Greenwood airvac cv2gip but Expelair, and other manufactures do them. I doubt if you have a flow meter but the custom settings on the usually about right.
Another thing worth mentioning, is how is the ducting run ? if its run in a non-insulated area then you could find that you have a bigger problem with condensation dripping off the ducting on to the ceilling. This will continue to happen unless the ducting is insulated.
The firm i used to work had to go and rip down the ceilings in a load of social houses that were less that 2 years old.
Everyone had black ceilings due to unisulated ducting.
Horrible job, i hate ventilation but the company got good money for if.