Interestingly, as Nigel has pointed out, the yew in the churchyard at Alton Barnes is younger, though the church is older (Saxon) than the church at Alton Priors. That seems to indicate (along with the stones under the Alton Priors church floor and the 1700 year-old yew) that Alton Priors is the older of the two sites.

Think we may have had this discussion in the Circles under Churches thread but the above might indicate the presence of a 'Celtic' people living and worshiping at their own sacred site at Alton Priors (there's a spring and stream in the same field as the church, and the Ridgeway is very close by), while right next-door at Alton Barnes there were Anglo-Saxons worshiping at their own sacred site - both sites eventually becoming Christianized.

Anyone who hasn't been to Alton Barnes/Priors might be surprised to see how close the two churches are - literally a five minute walk along an interesting stone path that links both churches together.