I eventually got to see the light in question yesterday evening. It's not part of the group controlling traffic on the South Circular travelling up past the station but is, in fact, one of a pair of lights controlling the pedestrian crossing across the end of Dartmouth Road.

It should have a louvre on the green light so that it can only be seen by vehicles directly in front of it - traffic waiting at the station/fish & chip shop lights shouldn't be able to see the green (or amber) light, only the red one.

So the light IS phasing correctly, but the louvre is missing.