“Nige” was a colleague of Dot in series 1, a dodgy copper who's bitter and twisted due to being disabled by a leg injury (although that leg injury is a lot less bad than it was and he now seems to be faking it to keep up the perks of getting disability benefits, as indicated by the moment when he runs off from a conversation with Cottan in the final episode). Plus he’s also resentful of people like Dot getting promoted above him. Morton makes money on the side taking bribes and sold the story about Dryden’s wife. Guessing this, Cottan blackmailed him into identifying Cole as “The Caddy”, deflecting suspicion away from Cottan himself.