My neighbour's flat smells, and the smell is now coming into my flat. What can I do?

By all means report it to the landlord. You should have done that long ago. There is no reason whatsoever for you to put up with it. If you report to the proper person and nothing is done, I would think you hae a legal action. Start with the health dept. Hope this helps and makes the smell go away or better yet, the nasty people.