In message 2012092509215558199-nospam@nospamcom, at 09:21:55 on Tue,
25 Sep 2012, eastender remarked:
Got to fly from Gatwick next week and rather than take the car to the
airport as usual I see I can get the ELL to New Cross Gate and change
and get there in about an hour. Question is on fares – National Rail
is showing a chapest fare of £19.60 from Dalston but if I got to say
Farringdon the fare is just £9.90. Is it possible/best to use Oyster
to New Cross Gate and buy a single from there?

The issue here is that the £19.60 fare is "Any Permitted" (including
tube transfers), but the £9.90 one is "Not Underground" (so in effect
"FCC only"-lite). If you can accept that restriction, then by all means
split the fare at Farringdon.
--
Roland Perry