At about 12:20 today, I saw an Overground train crossing the viaduct that goes over the South Western Mainline heading away from Battersea Park towards Wandsworth Road. My question is: Why was a train on that stretch of line in the middle of the day when there is normally only a limited service at the beginning and end of the day?
The 1159 train from Wandsworth Road to Clapham Junction didn't make it to Clapham Junction according to Real Time Trains, and the return service started from Wandsworth Road (not Clapham Junction) at 1221, so maybe it reversed at Battersea Park to stay off the running lines before it started its return journey? RTT states it made it across Factory Junction but not Longhenge Jn, so that would add up to give what you described. Strange given it only arrived at Wandsworth Road 3 minutes late - it'd be interesting to find out why this move took place.
It appears that the 1150 arrival from Dalston must have developed some sort of fault at Clapham, as the 1200 departure was cancelled. So the following train would have been diverted to Battersea Park while they decided what to do with the failed train to avoid trains queuing up outside Clapham.
There was some disruption today (Thursday 11th Oct 2018), I do not know the exact details but anyway, I was at Surrey Quays at 11:05 and the train describer said that the next train to New Cross had been cancelled and that a train to Battersea Park was due in a few minutes.