OpenStreetMap is correct. Cycling towards Millennium Wood the sustrans turn left 66 sign is on a lamppost near https://www.openstreetmap.org/node/306573805 after the start of the foodway https://www.openstreetmap.org/way/670616280 (that is really just a sidewalk in front of houses) and immediately before https://www.openstreetmap.org/way/28293574 
I would certainly not cycle on the footway https://www.openstreetmap.org/way/670616280