Smell coming from en-suite...

Hi, any advice would be greatly appreciated. I've noticed a soil/waste smell coming from the en-suite for a few months. Most of the time it's quite faint but on occasions it can be quite noticeable. I think I've narrowed it down to an open stack pipe which vents directly into the loft above the en-suite. I've included a photo of the open vent. Should this be connected to the roof vent which is above and to the right of the top of the stack pipe?
In the photo there also a green flexible connector pipe in netting just hanging from one of the rafters which looks like it been there from when the house was built.

Can't believe my eyes, I'd connect that open soil vent pipe to the roof outlet immediately.

Hi Jajodu, thanks for the quick reply. It's been like that for years, it's tucked away in the far corner in the loft.
I only noticed it when I went into the loft for something else. I'm on my way to get it done.
Thanks.

I reckon that the plastic piece in the string bag is the joining bit that someone forgot to put on.