Success
Basically:
Used ImgBurn 2 to build the iso using existing authored files on HD,
chose the layer break manually (probably using existing ifo),
saved iso and mds,
used ImgBurn to burn mds.
DVD DL played in Pioneer DVR111D and standalone ARCAM DVD player.
Thanks again to everyone who helped.

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ADDENDUM
Further improvement:
If you have the original DVD, load it in the DVD-ROM and open ImgBurn, go to Mode > Verify. On the R it'll show you exactly where the layer break occurs (right at the bottom). Something like:
QUOTE
Layer Information:
Layer 0 Sectors: 2,019,648 (53.16%)
Layer 1 Sectors: 1,779,871 (46.84%)
Then you can compare this with what shows up later during Build and in addition, use IfoEdit to have a look at the corresponding PGC and Cell IDs.