On 1 April 2015 the boundary changed so that at the north end of the parish the boundary follows the railway line, and the area north of the railway line is part of Sutton Courtney Parish.
The boundary before the change is shown below.
See also the OCC Oxfordshire Local View pages, specifically the icons for displaying various boundaries (Parish, District, County, Parliamentary), Post Code Units and much else.
District Ward Boundaries
Here’s the map of the new Blewbury and Harwell Ward, which returns two district councillors.
Click to enlarge the map, and you may spot that the Blewbury and Harwell Ward does not include the houses at North Drive in the Harwell Campus. These are part of the adjacent Hendreds Ward, so the Councillor for the Hendreds (just one), as well as representing the Hendred parishes will also represent the 70 or so houses on the Harwell Campus.
The consequence of this district change is that Harwell Parish is split into two parish wards (just as Didcot Town Council is split into six wards). The Harwell Campus ward returns 2 councillors, while the rest of the parish, including the new houses in GWP, returns 9 councillors.
It also appears that the Church of England Harwell Parish boundary matches the civic boundary, so this map also reflects the Parish boundary.