Crock - the reason that I'm offering the advice I'm offering is that the Southeastern website does not show special offers for advance tickets. Last month my advice was to go to the Southern website and buy advance tickets back from Brighton (provided that you could choose a train) at a price of £3.70. I then went on to say that if you couldn't pick a train then you might go 'Groupsave'.
Now, if you buy an advance ticket and are then 'persuaded' not to use it, I'd suggest that this was your decision, unless, of course, you were tortured, or plied with sufficient beer to render you incapable (and, if the latter, you might want to factor in the cost benefit if such plying). You might have tried to sell on the ticket, and post FNRttC breakfasts now host markets in reserved tickets because there are some very clever people buying up the advance tickets and then selling them on at cost - but not, as far as I know, this month!