A pragmatic group sequential, placebo-controlled, randomised trial to determine the effectiveness of glyceryl trinitrate for retained placenta (GOT-IT): a study protocol by Denison, F, Norrie, J, Lawton, J, Norman, J, Scotland, G, McPherson, G, McDonald, A, Forrest, M, Hudson, J, Brewin, J, Peace, M, Clarkson, C, Brook-Smith, S, Morrow, S, Hallowell, N, Hodges, L, Carruthers, K