How to do VPN on Demand completely free of charge

I was tasked with building a new VPN system in order to implement VPN on Demand and thereby ensure all traffic from mobile devices particularly iPhones and iPads was secured even when using public free WiFi hotspots.

Normally this would require purchasing a suitable VPN server e.g. Cisco, SonicWall, Juniper etc. which would entail a considerable cost however I have now managed to do this using free open-source components.

Details of this are available here How to do VPN on Demand for iOS at zero cost despite Apple's best efforts to prevent this