Thanks, Thomas, that's a great help. Although I haven't got it working yet, I think I understand what's needed--
1) I set the Type flag NS_EXTENDED (only)
2) I make up an array of Tags "Ext_tags[2]"
3) the first one is TAG_USER, SA_LikeWorkbench
4) the second one is TAG_END, 0
5) I put &Ext_tags[0] into the Extended field in ExtNewScreen
I call OpenScreen((struct NewScreen *)ns_ext), having to cast the ExtNewScreen struct to a NewScreen struct.
As I said, I've got that far and it still opens a lo-res screen, not a Workbench clone, but I'll persist for a while yet.
tony