The Greater London Authority's planning requirement for 10% of building energy to be obtained from renewable sources has led to extensive development of open-loop ground source heat systems in central London. Such systems are used for building heating and cooling with varying energy loads and balances over an annual cycle. Numerical groundwater flow and heat transport modelling is an essential tool for predicting the hydraulic and thermal impacts of proposed open-loop ground source heat schemes on existing licensed abstractions, other open-loop ground source heat systems, or other potential receptors, such as surface water ecosystems. Numerical models can be used to predict whether thermal interference is likely to reduce the efficiency of a system over time, and can be used to investigate possible alternative solutions. This paper focuses on the role of numerical heat transport modelling in the London Chalk aquifer. Conceptual models and numerical modelling strategies for the fractured Chalk aquifer are discussed. Case studies are presented and the results are discussed in relation to improvement of numerical modelling results and reliability of numerical model predictions on sustainable long-term ground source heat scheme performance, and assessment of their impact on other groundwater users.