Just finished rewatching Series 3 and it's such a goldmine of clues for this one!
Gill Bigelow hand in hand with Cottan at every step - the dodgy report, the golf tee in the envelope.
Patrick Fairbank - serving a prison sentence in HMP Blackthorn, where the leak came from!
Gill trying to insinuate her way in with Ted.
Ted's line to Gill - "There is a line between right and wrong, and I know on which side my duty falls." (THE line of the whole show)
Kate gaining prowess as a both a calculated investigator, principled, rule following etc.
My theory:
Kate is effectively 'undercover' as the DI, arranged by Ted. She has to maintain her principles and his cover by reporting what he's doing. It's part of the plan. Gill Bigelow tried to convert Ted, but when he delivered that line in S3, she knew it was impossible - it's only since then that any suspicion has been directed at Ted.
Steve may or may not be in the dark. He is loyal, but doesn't have either the emotional control or undercover experience of Kate. They can trust him to be law abiding enough as a whistleblower to go along with what they've planned though.
Ted may or may not end up behind bars at the end of this series. S6 would exonerate him though (alive or dead).