Ticket extensions
 

Yet again I've had hassles with ticket extensions. I really can't get
my head around the rules.

Arrived at New Cross Gate station on Wednesday 19th. Asked for a
return extension ticket from boundary Zone 2 to Gatwick Airport,
returning on Saturday evening. Clerk refused to issue one, saying
"it's not in my book".

I'm sure I've bought a similar ticket before; am I imagining this? If
not, why wouldn't the clerk sell me a ticket? Surely a return
extension to Gatwick from a Zone 2 station, coming back on a different
day, is not uncommon?

Thanks,

Patrick