Hi it obv. depends on when you are leaving. On my train at 7.50 not much chance I am affraid. But it is not that far really. I sometimes see people take fold up seats.
You can on the slow train, because that starts at sevenoaks, however, that takes about twice as long as the fast one that only stops at Orpington and London Bridge. Best option is to grin and bear it or move house.