Is anyone in a position to tell me - in a multi-tile object, what is the origin of the chief node in the mesh considered to be in terms of the tiles of an object? Is it the centre of the lead tile? The centre of the tile at 0,0? One corner of one of these tiles?

Or looked at another way, if I made an two tile object with in-world tiles at 0,0 and 0,1 and made 0,1 the lead tile (even maxis does not always use 0,0 as the lead tile) and all the position values in the CRES set to 0, and I then made a small mesh exactly on the axes origin point in Milkshape, how would the object sit on the tiles in the game?

Center of lead tile is (0,0,0). So you should move mesh in Milkshape accordingly.
If (0,1) as lead tile, and center of mesh at the center of (0,0) tile, the object would be shifted...um... backward, I guess. Because I have object with (1,0) as center, and it was shifted right.

So the mesh relates to the "lead" tile (the one flagged as lead tile in its OBJD)rather than the tile called "0,0" (if they are not the same tile)?

Yes. Figured it out through experimenting