As Jeremy has indicated he's content for discussion to continue, I'll share the thoughts I deleted previously. I posted a long time ago about the question of how David travelled from Piccadilly to Greenfield, and others have now picked up on this too. The documentary glossed over this aspect somewhat, and the feature on the BBC website is more informative. My own view is that David was killing time in Piccadilly, before meeting someone in a pre-arranged appointment at 1pm. I think that this person must have driven him to Greenfield, because I can't see how else he could have got there by 2, assuming that the landlord had the time right. I always thought it was a little odd that he would ask for directions without buying a drink - personally in that situation I'd order a half even if I didn't particularly want one - but if someone was waiting in a car outside, that would be a reason not to do so.
The location in which the body was found is another factor that, for me, points against suicide. Why would such a private person choose to end his life in such a (relatively) public location? Surely if that was the intention, you'd walk away from the main track to somewhere more secluded? But the biggest question is: why that particular location? If it had no particular significance for David (and I tend to think that his calling it 'the mountain' suggests it didn't), perhaps it did for his companion?
I think an open verdict was the right one, given the number of unanswered questions. I am surprised that the police have concluded that there was no third party involvement, especially as we now know that David paid for 5 nights in a hotel, as well as a return ticket to London. Surely that's a powerful argument against the suicide theory? The trip to Saddleworth was clearly purposeful​, even though that purpose is obscure. If this was a suicide, it must surely have been pre-planned; and yet there are too many factors pointing in the opposite direction to conclude that was what happened with any degree of confidence. Unless, of course​, he deliberately chose to manufacture an enigma, for reasons best known to himself. It's a little far-fetched, but not impossible, I suppose. I can imagine the idea appealing to a certain kind of personality, though perhaps more readily in fiction than reality. But there's so little evidence as to motive - for any given scenario - that any theories are almost entirely speculative.
