Recent developments in technology assessment and optimization methodology have seen an expansion in the use of Virtual Clinical Trials (VCT) as an alternative to conventional clinical trials. However, the ultimate value gained from VCTs relies on the speed and quality of results generated from the VCT pipeline. In many cases the end-point human observer represents a bottle-neck due to resource and time limitations. This motivates the development of a machine-based observer for key task-based assessment studies. Previous work using Deep Learning for detection and observer studies has shown significant promise, but requires large amounts of data for training. We therefore have built a model observer based on the VGG19 neural network architecture combined with transfer learning to successfully train a TLMO (Transfer Learning Model Observer) that can detect both screen-detected malignancies and simulated lesions in images of 303 x 303 pixels. Our results demonstrate a strong response for the detection of simulated lesions, 4mm in diameter, using the OPTIMAM VCT Toolbox, achieving a sensitivity of 0.78 and a specificity of 0.92. The model has also been tested using well-defined and ill-defined screen-detected masses where it achieved a sensitivity of 0.85 and a specificity of 0.83.