If you cut the Hastings Line peak fares by a third you'd be undercutting fares from Tonbridge and Sevenoaks where demand has largely recovered. If you've ever commuted on the classic lines since HS1 you'd know that your country service starts to brake at Grove Park so it can come to a Halt at a Red Signal for 5 mins to let suburban services cross in Front. You then go signal to signal until past St John's where you Jog to London Bridge for the post Thameslink 2 track section to Waterloo East. If you ran fast from Tonbridge you'd just wait at the Lewisham Signal for 10 mins!
Alternatively, you could have fares increasing beyond Tonbridge and Sevenoaks by much smaller increments so that they're frontloaded to the London end (but not undercut) but you still get an increasing discount the further out you get. Off-peak period returns in the area seem very hefty for the classic lines (over thirty quid for a day return from London to Ashford !).
With regard to Grove Park, perhaps they could shove more of the suburbans along the slow lines instead.
