Incidence of sight-threatening diabetic retinopathy in an established urban screening programme: an 11-year cohort study by Cheyne, CP, Burgess, PI, Broadbent, DM, García-Fiñana, M, Stratton, IM, Criddle, T, Wang, A, Alshukri, A, Rahni, MM, Vazquez-Arango, P, Vora, JP, Harding, SP