Thanks Parwaz7862 for reply and welcome!
I really like my new Dyson, first time I have one.
About the flap, the behaviour you describe is weird for me. I mean, if I let the vacuum cleaner horizontally (I use to do that), then very often the flap stays open, and I think that is not normal, because dust or bugs could escape from the assembly bin (it should be hermetic).
Anyway, if you confirm there is no spring, I believe you, it's ok, I will not buy a spare part.
Thanks a lot.