Silbaby 17
Image by nigelswift

but it looks like Silbaby is at least 100 m south of a line between the centes of the Sanctuary and Silbury

Not sure what you're looking at but no, its right on the line, you can see it on maps and in the flesh. The only question is by what degree of error, and that looks like a matter of a few feet at most and possibly none.

You can't beat seeing it in the flesh , and that is what mattered to the builders not maps or computer programmes . Using software and accurate grid refs can be more telling than a 1:25000 map and that is why a 10 figure ref for Silbaby would be useful , we might be in a position to see exactly how close it is to the Silbury - Sanctuary line and from that what margin of error .