I have read some of the theories on the board about The Mole being Katie, Darwin, etc. I can't understand how The Mole could not be Bill.
1. We have the telegram in Italy with the letters that spell out "THE MOLE IS BILL." I have every episode on tape. I went back and checked, and it is not a hoax. The numbers really do spell that out. The odds of that phone number by chance happening to spell that are are like winning the lottery.
2. In the first episode, we have BANE coming up as Bill's information was coming up. Again, I checked my tape, and it was really there. The odds of that happening are also pretty slim.
If you agree that the odds of both those occurrences happening by chance are beyond the realm of possiblity, we only have one other scenario in which The Mole could be someone other than Bill. That is if the producers put that in there to throw up off. That, in my opinion, is unlikely. Those are very much obscure, hidden clues. Anderson says at the beginning of every episode something like, "Watch for the clue to the secret identity of The Mole." Those are hidden clues. Take, for example, Bribs holding up The Mole Killer in an early episode. That is very obvious. That could not be a clue because it is too overt. The producers would put something like that in there to throw us off. But, to put something in that 99.9% of viewers AREN'T going to notice just to throw us off? I don't think that is a possibility.
If Bill is not The Mole, I'll eat my hat. (I don't own a hat.)