An axial flow turbine diaphragm comprising an annular array of blade units, each blade unit comprising: - an aerofoil; - radially inner and outer platforms integral with the aerofoil, the radially inner platform consisting of a segment of the inner diaphragm ring and the radially outer platform consisting of a segment of the outer diaphragm ring, at least the outer ring segment comprising engagement features (161, 162, 165, 166) that mechanically engage with complementary engagement features on neighbouring outer ring segments in the annular array of blade units, the engagement features acting to interlock neighbouring outer ring segments and produce a self-supporting turbine diaphragm, wherein the engagement features on the outer ring segment of each blade unit include hook features (161, 165) on both circumferentially facing sides (163, 164) of the outer ring segment that engage with complementary features on neighbouring outer ring segments of adjacent blade units, the hook features being oriented to maintain axial location of each blade unit relative to its neighbours wherein the engagement features on the outer ring segment of each blade unit include tongue and groove features that engage with complementary features on the outer ring segments of adjacent blade units, the tongue and groove features being oriented to maintain radial location of each blade unit relative to its neighbours, characterised by the tongue and groove features comprising: (i) a groove on a circumferentially facing first side of the outer ring segment, the groove being formed as a gap between a radially outer part of a hook and a radially outer, circumferentially projecting lip portion of the outer ring segment; and (ii) a circumferentially projecting tongue projecting from a circumferentially facing second side of the outer ring segment in exact opposition to the groove on the first circumferentially facing side. - an aerofoil; - radially inner and outer platforms integral with the aerofoil, the radially inner platform consisting of a segment of the inner diaphragm ring and the radially outer platform consisting of a segment of the outer diaphragm ring, at least the outer ring segment comprising engagement features (161, 162, 165, 166) that mechanically engage with complementary engagement features on neighbouring outer ring segments in the annular array of blade units, the engagement features acting to interlock neighbouring outer ring segments and produce a self-supporting turbine diaphragm, wherein the engagement features on the outer ring segment of each blade unit include hook features (161, 165) on both circumferentially facing sides (163, 164) of the outer ring segment that engage with complementary features on neighbouring outer ring segments of adjacent blade units, the hook features being oriented to maintain axial location of each blade unit relative to its neighbours wherein the engagement features on the outer ring segment of each blade unit include tongue and groove features that engage with complementary features on the outer ring segments of adjacent blade units, the tongue and groove features being oriented to maintain radial location of each blade unit relative to its neighbours, characterised by the tongue and groove features comprising: (i) a groove on a circumferentially facing first side of the outer ring segment, the groove being formed as a gap between a radially outer part of a hook and a radially outer, circumferentially projecting lip portion of the outer ring segment; and (ii) a circumferentially projecting tongue projecting from a circumferentially facing second side of the outer ring segment in exact opposition to the groove on the first circumferentially facing side. (i) a groove on a circumferentially facing first side of the outer ring segment, the groove being formed as a gap between a radially outer part of a hook and a radially outer, circumferentially projecting lip portion of the outer ring segment; and (ii) a circumferentially projecting tongue projecting from a circumferentially facing second side of the outer ring segment in exact opposition to the groove on the first circumferentially facing side.