Osma-vent leak?

Hi,

I live in the top floor of a block of flats (second floor) and in the loft there is a black pipe of about 100mm (at a guess) which has a quite cap on the end of it which says on it "Osma-vent".

The "osma-vent" is dripping in the loft, I'm not sure where the black pipe leads but I'm told it could be a stench pipe as there are no stench pipes on the outside of the building.

I do not understand where the water is coming from as obviously to get into the loft its got to come quite a way up hill....unless the water is just condensation, but then why would the osma-vent have just started dripping now

Do you guys know of any known issues with the osma-vents or have any ideas what the problem could be?

Any help greatly appreciated

Thanks

Rich.