VaTam, are you still living in the UK? If so, you could try the Public Health Dept. at your local council office. But it's possible your neighbour is lying inside the flat, sick or worse, in which case it will be a police job to break in to the flat. I'd try both the PH Dept. and the police - I wouldn't be happy to dispose of the parcel myself.
Usual disclaimers........IMHO, YMMV.