You cannot put one-day travelcards on Oyster, nor can you use Oyster
Pay-as-you-Go on most of National Rail. So, I haven't done the sums for
you particular route, but it is usually the case that Oyster is more
expensive than a paper travelcard for occasional journeys that involve
National Rail.
--
Paul Terry

I've got a consensus! Thanks, Paul. A definite travel card journey, then.

Regards,

Demiurge.