According to Royal Enfield India, several outside people were involved in the creation of the C5.
The engine was validated by Dr. Stuart McGuigan, formerly of Cranfield University in the UK. He started his career in BSA and Aston, then got a Ph.D. and joined the Royal Military College of Science (now Cranfield U) where he became Director of Design in the Engineering Systems Department. He was involved in the design of the five speed gearbox of the Bullet Sixty-5 and also in the HDT Diesel motorcycle for the US Marine Corps (he's riding it in the picture). He was helped by Ricardo PLC, a consulting firm in the UK. The design team has been in collaboration with, Xenophya Design, UK. If you go to their gallery you will see they were involved in Mac Motorcycles, the electric TT bike, Derbi scooters and other projects. In all cases they include their sketches, except in the case of the C5 where they only display the final product. Obviously they were in such awe of what had been created!
The chassis was developed by Vepro in the UK and Engines Engineering of Italy (now property of Mahindra & Mahindra, India). The fuel injection was developed with Keihin from Japan.