I reckon that's Phelan all over. Excelllent last minute thinking. He knew Rodwell was very suspicious of him hiding something in that backpack, so by putting a ring in there in case of some hassle, then Rodwell finds the ring, as if Phelan was saving it for later. Phelan proposes to Eileen. It saves Phelan's bacon and makes Rodwell look stupid all in one go.
I remember months ago we had a scene where Phelan managed to get at the CCTV recording in the builder's yard and remove incriminating evidence of him up to no good. He's a master at covering his tracks, and we all love to hate him for it.



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