Constructing tunnels in highly congested urban cities is a challenge, as the construction will inevitably take place in close proximity to existing structures both above and below ground. As more tunnels are being constructed, it is inevitable that some tunnels will be constructed in close proximity to existing tunnels and monitoring of the existing tunnels in this case is paramount for the design and construction works. The discrete nature of conventional monitoring instrumentation requires significant interpolation and judgement in order to understand the overall behaviour of the tunnel itself. Subsequently, conservative design approaches will have to be adopted to cater for the gaps in knowledge, which could lead to unnecessary delays and high costs. Distributed fibre optic strain sensing systems based on Brillouin Optical Time Domain Reflectometry (BOTDR) could offer an alternative; in this paper a case study is presented where fibre optic cables were deployed to monitor the response of the cast iron Royal Mail tunnel in the vicinity of London Liverpool Street Station during the construction of Crossrail’s new platform tunnel directly below it. Single mode single core tight-buffered cables were attached directly to the intrados of the cast iron tunnel lining of the Royal Mail tunnel to understand its response during the construction works. This paper focuses on the challenges and considerations in deploying the fibre optic system in the tunnel and presents some of the data, which demonstrates the potential benefit of using such a system in real, complex tunnelling scenarios.