We describe a model that estimates the social benefits of carbon (C) sequestered in plantation Sitka spruce in Great Britain. Final net present values (NPV, base year=2003) resulting from plausible variations in model parameters are calculated. The discount rate, social value of C, timber yield, rate of gain into live wood, length of rotation, lifetime of products, amount of C displaced by products and the changes in C flux on afforested peat soils are the most influential model components. The study predicts that C fluxes in actively managed forests in second or subsequent rotations or planted on peat soils will tend to have low or (on average) negative NPV.