Make sure air can get into the bathroom as well as out through the fan. Possibly insulate the ceiling if you can get access.

I agree with the above.

You need 'passive intake' as a balance to a fan thats extracting.  If it just sucking ona vacume, it will not extract.  If it is sucking and pulling new air into the room, it will shift the moist air out.

Taking a little of the bottom of the door, or fitting a grill to a wall or door will allow air into the room when the extractor is running.

Mould is caused by damp air condensating on walls and ceilings.  A cold wall or ceiling will cause the moist air to condensate.  Insulating the walls and ceiling well will help this issue.

You obviously also need to ensure you air the room - open windows regularly.  Keeping the room warm helps too.