My other suspicion as to the cause is that a pedestrian crossing phase has been added to the lights which means cars on the south circular are being held on red for a lot longer than previously.

It might be that at weekends in particular more pedestrians are using the junction for example to get acrosss to the Horniman or use the small park on the other side.

Whatever, it does not seem to be working well because it was still causing a traffic problem eastbound at 8pm last night.

Email being drafted to TfL......