Thanks andrewr, that does say that £1.80 / £1.50 are the fares for zone 2-3 travel on a 'through' national ratil / overground journey.

I suppose the big question is whether post 23 May Forest Hill will be a national rail station or both a national rail and overground station?

Also, although it does specify that you pay oyster underground fares on routes that accepted oyster before 2010, it's not entirely clear what happens on routes that didn't exist before 2010.

Hmmm.