An inflatable flood defence structural unit (10a, 10b) comprising: at least one inflatable chamber comprising a front wall and a rear wall which extend in a substantially parallel orientation, at least one of the front and rear walls (12, 13) providing a water barrier,, a first inflatable buttress which is rigidly bonded at a proximal end thereof to the rear wall of the chamber, proximate a first end of the chamber and which extends in a direction which is away from the chamber, a second inflatable buttress which is rigidly bonded at a proximal end thereof to the rear wall of the chamber, proximate a second end of the chamber, and which extends in a direction which is away from the chamber, the first and second buttresses (18, 19) further comprising side walls (20, 21) which couple the proximal end of the respective buttress to a distal end of the respective buttress, the side walls (20, 21) of each buttress (18, 19) separately comprising an inner side wall (20a, 21a) which faces inwardly of the unit, substantially toward each other, and an outer side wall (20b, 21b) which face outwardly of the unit, substantially away from each other, the unit (10a, 10b) further comprising a fastening arrangement disposed upon at least one of the first or second buttress (18, 19) for fastening the first and second buttress (18, 19) to a second and first buttress (19, 18) respectively, of a further structural unit (10a, 10b) characterised in that the at least one inflatable chamber comprises a first and second end wall (14, 15) which couple the front and rear walls (12, 13) of the at least one inflatable chamber together at the respective first and second end of the front and rear walls (12, 13) of the at least one inflatable chamber, and wherein in use, the outer side wall (20b, 21b) of the first and second buttress (18, 19) is arranged to contact an outer side wall (21b, 20b) of the second and first buttress (19, 18) of the further unit. at least one inflatable chamber comprising a front wall and a rear wall which extend in a substantially parallel orientation, at least one of the front and rear walls (12, 13) providing a water barrier,, a first inflatable buttress which is rigidly bonded at a proximal end thereof to the rear wall of the chamber, proximate a first end of the chamber and which extends in a direction which is away from the chamber, a second inflatable buttress which is rigidly bonded at a proximal end thereof to the rear wall of the chamber, proximate a second end of the chamber, and which extends in a direction which is away from the chamber, the first and second buttresses (18, 19) further comprising side walls (20, 21) which couple the proximal end of the respective buttress to a distal end of the respective buttress, the side walls (20, 21) of each buttress (18, 19) separately comprising an inner side wall (20a, 21a) which faces inwardly of the unit, substantially toward each other, and an outer side wall (20b, 21b) which face outwardly of the unit, substantially away from each other, the unit (10a, 10b) further comprising a fastening arrangement disposed upon at least one of the first or second buttress (18, 19) for fastening the first and second buttress (18, 19) to a second and first buttress (19, 18) respectively, of a further structural unit (10a, 10b) characterised in that the at least one inflatable chamber comprises a first and second end wall (14, 15) which couple the front and rear walls (12, 13) of the at least one inflatable chamber together at the respective first and second end of the front and rear walls (12, 13) of the at least one inflatable chamber, and wherein in use, the outer side wall (20b, 21b) of the first and second buttress (18, 19) is arranged to contact an outer side wall (21b, 20b) of the second and first buttress (19, 18) of the further unit.