How about Mrs Hughes? She seemed positive that Mr Bates and Anna had nothing to do with it. How can she be so positive?
Because she knows it is not in Anna's character and she learned from Anna about the unused ticket proving Bates did not go to London that day.
Furthermore, Mrs Hughes was at Downton that whole day. If Green was murdered, it has to have to have been someone who was IN LONDON that day. Gillingham, Mary, and Anna were certainly all in London at that time, but as Bates pointed out, there were millions of people that day.
The fact that a witness identified Anna but had doubts later it was her is itself revealing: another possible victim perhaps who resembled Anna, given that Green's victims seemd to always be of a similar height and build according to Vyner.
I hate to imagine that we'd be left hanging, but there is still a possibility that no one killed him. Just because he spoke to someone on the sidewalk doesn't rule out what was first believed; that it was an accident and he just fell. Not as exciting, but still possible.
This always bothered me about Anna getting arrested because she has motive and a "pattern" of attacking men (if they could actual prove that she did it more than once!) and there was a "witness." Yet on that busy street no one witnessed him actually being pushed. It's like those cheesy detective shows where they have next to no evidence, but they string together a theory and the murderer just shrugs his or her shoulders and confesses!
What do you think?