Nene Valley Railway Edmondson tickets

Does anyone know how to get issued NVR Edmondson tickets? Week before last I made a return journey from Ferry Meadows. The office was staffed and a large assortment of Edmondsons on view. I worked out that with myself and child I could get four different tickets. I explained to the clerk how much I wanted the tickets and was prepared for this more expensive option than a day rover. He took the money and still issued me a ticket from a pad with an Edmondson souvenir! I was furious.