"MIG" wrote

It has been suggested, although I would dispute it, that the
travelcard part can't be used after the return journey has been
completed. I can understand that it's only valid for one return
journey outside the zones, but I can't see why one couldn't get a lift
back to London and continue using the travelcard (other than due to
not physically having it).

It is certainly permitted to use a ODTC from Dartford (outside the zones) to
return there, and then use it on a London bus (Travelcards are valid on
buses which form part of the London bus network, even outside the GLC
boundary). It may be necessary to explain this at the ticekt barrier, and
not put the ticket into the gate.

Peter