Thanks for the figures island.
If East Coast services are indeed desired, for an even cheaper option, Spalding- Reading advance singles for 7 April currently appear to be available on the 09:02 from Spalding (arr. Reading 12:48) and the 18:02 from Reading (arr. Spalding 20:54) for £20 each way.
With these tickets, stopping short at Peterborough would not be permitted; if you wished to finish your journey there rather than continue to Spalding, you would need a Reading- Peterborough advance single on the 18:02 (arr. Peterborough 20:19) at £17.50 instead.
If it was my money, and I didn't have any railcard, I'd probably buy:
Spalding- Peterborough Off Peak Day Return rte Any Permitted £7.50
Peterborough- London Zones 1-6 FCC Only Super Off Peak Day Travelcard £24.00
(which, as I understand it, is valid on any operator's service within the Travelcard zones)
Boundary Zone 6- Reading Off Peak Day Return rte Slough £10.50
Total: £42.00