"I would have thought that the two places you would put them for 'ritual' reasons would be at the entrance and behind or under the altar (i.e. round the back)."
Fair enough, and Littlestone's posting seems to imply that might well be the pattern.
Only my thoughts of course. However, I come across dirty great boulders in the middle of north and south walls in early churches, so I ain't got a clue why they could be there other than 'they were there so we used them'. Perhaps I should start taking more notice of these.