They got the name slightly wrong ... G-21-3 was a prototype Napier Agricopter, derived from the Bensen B.SM (B.SMEJ) Gyrocopter. Also known as the Napier-Pan Britannica Agricultural Gyroplane and the Napier-Benson Gyrocopter.
Three Agricopters were built in 1961 (likely all at Luton): G-29-2, G-29-3, and G-29-4.
http://www.nwamuseum.co.uk/AIRCRAFT%20MANUFACTURE-1.pdf
Aircraft Manufacture in West Essex, North Weald Airfield Museum , 2010, pp.11-12