I would definitely recommend using realtimetrains in that type of situation, as its information appears earlier than it goes up on the monitors or on the live departures site.

It's also good for finding out what platform the LB trains will leave from before they appear on the monitors, though they sometimes make a late change.

A friend who lives in Streatham recently spent half an hour waiting for the 17.52 to Victoria to turn up, as the monitors progressively increased the delay until it disappeared at about 18.20. I got on this train at LB and it was always listed as terminating at Sydenham. So clearly their communication systems are not up to scratch somewhere.