The alleged grass was Paul Roundhill, whose flat they were in. The person who I have it on good authority threw the victim out the window, was Doherty's minder Johnny 'headlock' Jeannevol. He's the one that confessed to the crime at the police station but later retracted his statement, saying he was on cocaine.
I am in little doubt that both Doherty and Roundhill know this is what happened but have kept quiet about it. The Met are a disgrace for not investigating thoroughly and allowing someone who confessed to the murder to just get away, due to the reasons he gave!