I was told that there would be single file traffic on the south circular with temporary traffic lights. The bit to be dug up is between Waldram Place and Westbourne Drive, which is why they have closed off Perry Vale, Westbourne Dr and Stanstead Rd. The contractors have been waiting two days for the TfL guys to switch off the lights at the junction of Perry Vale, otherwise the work would already have started.
Once they have figured out where to join the mains onto the pipe under the S Circ (apparently the plans are a little vague!), they will continue up Perry Vale to join onto the pipe outside the station.
You can currently get into Stanstead Rd via the Co-op car park.