We used the 171 to go to New Cross and picked up the Overground from there on Sunday.
For the return journey we went to Canada Water and tried to use the replacement bus service. It was a shambles - poor signage, inconsistent announcements and when we eventually found a rail replacement bus, it was terminating at New Cross Gate, where a bus full of people was decanted onto the narrow and crowded pavement to wait for a second rail replacement bus (for the same line, remember).
Genuinely surprising given the generally high standard of service from the Overground, but I guess they don't have that much experience (yet) of running rail replacement buses.