Just topped up at Forest Hill. Thanks rbmartin for list of Southern stations but would have to make special trip (with Oyster fare) to collect at those - still useful to know.

Euan, should work in principle for any National Rail Voucher but can't vouch for that as have only managed this with Southern.

Man at ticket office was startled that I had managed to get so much out of Southern, in particular, for a single journey and discussion then ensued about how few people pursue this. Get claiming people! The train operating companies claim for delays from Network Rail anyway.