(In practice, they usually go 'no record of any delay' / 'passenger action' / 'act of ghod' / 'snow in winter is freak weather' etc and turn it down or sometimes - as with my Central Line refund claim - just ignore it entirely.)
But what is the cost of a single journey, say in zone one now?
Is it the £2 cash fare? The £1.60 Oystercard (so named, because it's difficult for them to be open about the problems) fare? The £1.50 2004 carnet fare? Or the £1.15 2003 carnet fare that was actually paid for the ticket used that morning?
Turns out - I got a refund voucher this morning - they're going for the £1.60. Hmm.
Oh, speaking of the difference between the 2003 and 2004 carnet fares - the unspeakable people at the 'advance tickets' service received my work order for fifteen books of (ten) carnets before Christmas. On the 6th Jan, two days after the fares were increased, they ring while I'm out of the office to say 'did we know you can only get carnets in books of ten, and fifteen isn't a multiple of ten?' They get faxed back with 'yes, that's why the order says "fifteen books of ten"'.
Tickets finally arrive - they've charged us the 2004 price, claiming the order was made on the 8th Jan. This is, as they say, being disputed.