Thanks for starting the thread, fruitstick. Marking my place for later 
Final predictions:
Swimming against the tide here, but I still think it will have been Nige. He's very focused on how he can make money because working for Mark isn't that great a moneyspinner, so what else has he been up to - in addition to the poaching/diesel theft? Also - he pinched the diesel for the van the night before - so why was he so late the next morning, when Mark himself was running late? I can't get over the fact that he offered Susan £500 to get lost when that also is the amount that Danny had hidden in his room. Was that their "cut" from something they had been doing or was Danny hiding the money for Nige? There seems to be a hint they came about the money in the same way.
I think he killed Danny in a fit of anger. Perhaps Danny found out that Nige knew about his dad and Becca. Danny was going to confront his dad about the affair, but that would have let the cat out of the bag that Nige had blabbed to Danny. Nige doesn't want to lose his job, so he begs Danny to keep quiet. Danny, incensed, insists that he is going to tell and Nige loses his temper and kills him.
It may transpire that Susan cleaned up a lot of the evidence to help Nige.
The scene with the postman will return. In that scene the postman seems to look around while he and Danny are arguing, almost as if he's waiting for someone else to turn up? Maybe there was a drugs ring and Danny was the perfect mule on his paper round?
Ellie may find out some uncomfortable truths about Joe/Tom. Perhaps Mark isn't the first married man Becca has had a dalliance with?
Interesting that they are promising us some sort of surprise at the end. I think it might be:
a) DT keels over and we are left not knowing whether he lives or dies
b) the wrong person will have confessed to the crime in order to protect someone else and we will get a glance from the real killer at the end, showing that Hardy still can't get his man
OR - and I think this really might be it:
c) Danny's murder is resolved in exactly the right way, but we cut to Steve, sitting playing with a pendant in his hand - the pendant from Sandbrook. He shoots a chilling look at the camera, and we fade to black.