Is it acceptable to allow the extracted air to run into the dead space between the ceiling and 1st floor between the joists. It's a 3 meter run before exiting through a flap vent on the outside wall. I removed the fan to clean it and was surprised to find no ducting attached. A second question; is there a vent that won't open in the gales we've been having and allow the rain in
No, don't even think about it, moisture will get everywhere. You can run ducting in the void however.