On 24 Oct, 00:08, Mr Thant
wrote:
No. Following the Routeing Guide rules, the only permitted route is
direct via Lewisham and Sidcup (the shortest route), or via a direct
train.

But that means retracing your route...which isn't strictly allowed.
You would have to continue your trip on a direct train. If you were
challenged twixt Greenwich & Lewisham, you'd get a penalty fare, or
excessed.

Indeed, at times of the week when train services are less frequent, it's
not uncommon for National Rail Enquiries to suggest travelling away from
London and changing at Dartford among its options for getting to London
from Crayford. Does this mean it is a 'valid route' for a Cheap Day
Return?

The journey planner has no official authority to permit a non-
permitted route. Does the red "you need to buy two tickets" warning
appear?

Have you noticed that tickets booked via the online NRE are now saying
that the ticket is valid for any journey displayed by the online
planner?

As a final twist, if you do break your journey at a particular station,
are you then obliged to resume your journey at that same station?

No, but it has to be further along a particular permitted route that
covers both legs. Switching from Lewisham to Greenwich wouldn't be
allowed.