Is a Spalding - Sleaford off-peak return valid via Peterborough and Grantham?
Taking a look at the Routeing Guide, you get as far as realising both stations have the common routeing point Sleaford which means it’s a local journey, so only direct trains, the shortest route or any route within 3 miles of the shortest route is permitted.
Without actually taking a look at the National Rail Timetable for mileages, I’m going to hazard a guess that Spalding - Peterborough - Grantham - Sleaford is a bit more than 3 miles longer than Spalding - Sleaford.
No easements I’ve spotted either, so I don’t think so, no.