Conducting cyber security research within live operational technology and industrial Internet of Things environments is, understandably, not practical and as such research needs to be undertaken within non-live mimics or testbeds. However, testbeds and especially those which are built using real-world infrastructure are expensive to develop and maintain. Moreover, such testbeds tend to be representative of a single industry vertical (often based upon the skill set or research focus) and built in isolation. In this paper we present a reference architecture, developed whilst designing and building the Bristol Cyber Security Group ICS/IIoT testbed for critical national infrastructure security research.