My wife has an all-zones Travelcard/Gold Card. Next Wednesday she needs
to make a single journey from Northfleet in Kent to central London,
leaving at peak time (8.23).

The train at this time travels to Dartford, which is immediately outside
Zone 6 and therefore still not covered by her Gold Card, then goes fast
to Abbey Wood, which is in Zone 4, i.e. quite a way into her Gold Card
area.

So my question is, can she buy a ticket from Northfleet to a
just-in-Zone-6 station such as Slade Green (which she'd be passing
through but not stopping at) and use that to bridge the part uncovered
by her ticket? Or would a ticket to Dartford plus her travelcard cover
her (I'm guessing not, since the stretch between Dartford and Zone 6 is
then not covered)? Or does she have no choice but to fork out for a
single from Northfleet to Abbey Wood, despite the duplication of ticket
validity? Any ideas?

Thanks in advance for any help,

Paul