There are very few fans sold for domestic use which will work effectively on 7m of duct. Those that do will tend to be excessively noisy and will most likely be inline centrifugal types, not the sort which mount in the ceiling.
7m of smooth wall duct will reduce the effectiveness of the fan by at least 50%. If using that flexible spiral ducting, it will be much less, possibly only 25% of the original rating.
As this is a bungalow, and the bathroom obviously has at least one external wall (with the window in it), surely the duct length could be reduced significantly?
You also need another vent so that air can enter the room. The usual method is to have a gap under the door, and position the fan as far from the door as possible, so you get a decent airflow across the whole room. For a 6 inch round fan, this would translate to a 25mm gap under a typical size door. (Normally it's about 10mm gap for a 4 inch round fan).
The fan position is critical - if the fan is located close to where air enters the room, it will be useless.
With a properly sized and installed extractor, it shouldn't be necessary to open the window - in some instances, an open window can actually reduce the effectiveness of the extractor, depending on the relative positioning of the window & extractor.
