Nailsea - Bristol
TM▸ SDS ^3.80
Bristol TM - Didcot SDS ^16.15
Didcot - Peterborough SSR ^39.35
Didcot - Nailsea SSS ^12.20
Based on 0713 out, connecting into 1000 from King's Cross, arriving Peterborough 1045. An earlier arrival in Peterborough can be had with this combination by taking the 0644 from Nailsea, connecting into the the 0930 from King's Cross arriving Peterborough 1020.
Return permitted at anytime, remembering that you have to catch a train from Paddington back to Bristol/Nailsea that calls at Didcot. No afternoon/evening restrictions into King's Cross or out of Paddington with a Didcot to Peterborough Super Off Peak Return (SSR). And equally no restriction on the Didcot - Nailsea Super Off Peak Single at the time you will be returning.
Total: ^71.50 (EDIT NOTE. Amended total after my maths was corrected by Ollie.

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Just a small saving on Ollie's suggestion. Best I've found.
