I'm with Tollers on this one !
this makes me wonder if my subconcious grammar usage has been influenced by LoTR - probably.
So for me-
1. Comparative of far: If you can bear your load no farther,say so. --H. Martineau. It was not thought safe for the ships to proceed further in the darkness. --Macaulay.
Either would do, 'farther' sounds more antique (perhaps from Bible usage?)
2. No notion of far: Down he sat without farther bidding. --Dickens. I now proceed to some further instances. --De Morgan.
Definitely 'further' - Who the Dickens would use 'farther' here?
3. Intermediate: Punishment cannot act any farther than in as far as the idea of it is present in the mind. --Bentham. Men who pretend to believe no further than they can see. Berkeley.
Still 'further', Bentham's sentence seems out of true for me.