Shortly after crossing the M4, heading north towards Seagry and Somerford, there is a bridleway to the left, with a fingerpost pointing up a slight hill. This is often quite wet, as water flows down the path when it has been raining. But turn left here and follow the track parallel to the M4. until you reach a gate into a field.
When the M4 route was being created, the parish had missed the opportunity to ask for the lane to Harding's Farm to be kept intact, either on an extra footbridge over the motorway, or in a tunnel under it.
It was not until after the M4 construction had started that this short stretch of footpath was created in order to regain the 'old road' to Harding's Farm.
You are now in Seagry. The land to the north of the motorway, and south of the Seagry-Stanton Road, was originally in Draycot Cerne and Sutton Benger, until the arrival of the M4 created a few sensible boundary changes.