Validation of the Oxford WebQ online 24-hour dietary questionnaire using biomarkers by Greenwood, D, Hardie, L, Frost, G, Alwan, N, Bradbury, K, Carter, M, Elliott, P, Evans, C, Ford, H, Hancock, N, Key, T, Liu, B, Morris, M, Mulla, U, Petropoulou, K, Potter, G, Riboli, E, Young, H, Wark, P, Cade, J