£33.75 is a normal Super Off-Peak return but you wouldnt arrive until 8.15. Its hard to find a way to get you into Peterborough by KO without paying a stupid amount and having to complicate ticketing.
Cant get ANYTHING cheap as you are of course leaving Kings Cross during the evening rush.
With my railcard, i can get it for £37 but split into 3 tickets from Eastbourne -
Eastbourne - London Super Off-Peak Travelcard
Kings Cross - Peterborough FCC Only Single
Peterborough - Kings Cross East...