Hi reefy,

For marking out the edge holes - although I've not used one, I've often thought about buying transfer or universal punches in case I needed to transfer holes:

Universal transfer punches or Transfer punch sets:
http://www.arceurotrade.co.uk/Catalo...unches-Scribes

With the holes drilled in the top surface of one plate you would align it to the edge of the other (using clamps, 90 degree blocks etc.) put the correct punch into the hole and 'transfer' the centre point onto the edge surface.