Hastings is mega dodgy. He didn't report his financial difficulties in series two, and he got caught up in the dodgy property deal in the first place. He shagged Gill Biggeloe. He issued a Farenheit order to Steve against John Corbett, when Corbett was going to expose who H was. He was adamant that Hilton was H and wanted to stop the investigation there. He meddles too much. He accepted the cash from the OCG; it may not have been directly from them, but he knows what's going on when you accept wads of cash in a brown envelope. Something's going on with Steph, even if it may only be him wanting to look out for them... although his motivation for that is because he had a thing with Corbett's mum which is in itself indication that he's dodgy because Corbett's mum was his informant.
I'm not sure he's H because I think that's just too obvious, but if he wants to catch bent coppers he could start by handcuffing himself