The Nene Park (aka Ferry Meadows) information board says:
One story maintains that they are draughts from arrows shot from Alwalton churchyard by Robin Hood and Little John; another claims that the stones, which are nicked in the tops like arrows, are a memorial to St Edmund who was killed by the Danes’ arrows. The most plausible explanation is perhaps that they mark a track to a crossing point on the River Nene, where the Abbot of Peterborough granted the Abbot of Bury toll-free passage for the stone being transported from the quarries at Barnack to build the Abbey of St Edmund.