Chris in 1066 and Lydart - according to the Canford Parish website it looks like this part of the building, the South Chapel, was built in the 14th C - so, yes it was a new addition to the building! (the church was originally Saxon)

I enclose the plan from their website, showing the age of the various sections, below.
Thanks for the link to the church plans site though, Lydart. I haven't had a chance to study the plans they have for Canford church yet, but it looks like it will be a generally useful site for other research also.
I haven't discounted the possibility that the stones had toppled and been moved - just that if they did it was a very long time ago as they've been against the wall since at least 1886!
Thanks also to goldy for the Budden family site link. Unfortunately they don't seem to be anything to do with my line - at least I haven't come across a link yet and I have mine back to the 16th C. Mine came from Ringwood to Canford in the 17th C - the website is for the Buddens from Loders which is in the west of Dorset, near to Dorchester.