In the Layers panel, select the layer or folder you wish to add to a second document. Choose Edit > Copy. Select the second document window and choose Edit > Paste.
That's what I ended up doing - I actually prefer it to dragging etc - thanks!
An extension which might be useful can be found at
It will copy and paste the layer hierarchy structure as well
the copy and paste will not work for layers hierachy that has sublayers.
it will simply ignore it and paste into the parent layer
Another workaround is to share entire layer and sublayer to a blank new page.
Export this page out as external FW file and import into another new document to maintain its hierachy.