Thanks Muss but I already know to ask for boundary stations - still narked (and waiting for a TFL response) on why they can't cope with third off PAYG fares for Gold cards since it is now more expensive.

Buses stopped being zoned about 5 years ago. I now buy a 1-2 zone Travelcard and hop on a bus to Denmark Hill to get the train to Victoria.

I think Billham is right on the 'must be valid in all zones travelled through' point although how does that work PAYG wise if you want to get on the London Bridge to Victoria service from Forest Hill since presumably it would you to have a valid zone for East Croydon?