Thanks ChasH.
So it looks like it was named after some anglo-saxon (or even Roman?) person called Hersa.
The reason I asked was because I saw Haresfield referred to as "Harres Feld" in a will in the 1500s of someone with the surname Harres. Seeing as most people only adopted surnames in the 1300s it looks unlikely that the owner of the will had anything to do with the naming of the place, and that it was just a coincidence (or even a bit of creative spelling by the clerk) that the person's name matched the place.