Have you tried rebooking i.e. getting a Cheap Day Return to Kings Cross Thameslink then Saver from London to Nottingham via Grantham (not obvious but this route is actually cheaper than using Midland Mainline). Anyone with a zones 1-6 travelcard season ticket can get a Saver from Hadley Wood to Nottingham also via Grantham (as one of the tickets is a season ticket I don't think get a train from King's Cross that stops at Hadley Wood - National Rail Conditions of Carriage C19(c))
Now try selecting via Going via and entering GRANTHAM. I think one can get a fare for £41.50. Also on if you use GNER's site - you might get cheap singles with a 20% discount not available on the trainline.
Comments
can get a day return pancras to nottingham for 47 quid. or book quick for certian times buy buying 2 singles and you can get it for about 25.