ok at last i’ve got to the bottom of this, the problem i was having was that when i used harvest to create a blueprint(static mesh prefab) i would end up with lots of new blueprints. this was caused be me including other blueprints (doors) in the selection.
so now i can select my meshes (ca 300) in one building then use harvest to create a blueprint.
as expected it was my workflow and not rocket
thanks for being so patience with me
btw can i add other blueprints to my prefab/blueprint eg doors, lifts, ladders