Have to admit, my Garmin, the Endomondo app on my phone (when I used it) and a route measuring tool I used all came out at around 5.17k so I've always thought it could be a little bit long. My GPS starting place though sometimes comes out near the traffic lights at the top of Warminster so who knows!
However, even if a re-measure shows that it is long, I agree with Waldo, it should stay as it is. There is a need to compare with previous times.
Just for information, I notice that the route shown on Graves parkrun course page both starts and finishes at the corner of the car park.