A heat exchanger (10) comprises a first sheet (12), a second sheet (14) and an additional sheet (30). The first and second sheets (12, 14) are wound around an axis (X), each sheet (12, 14) has hot and cold edges at the hot and cold ends (16, 18) respectively of the heat exchanger (10). The hot and cold edges of the first sheet (12) are joined to the hot and cold edges of the second sheet (14). The end (12A) of the first sheet (12) is joined to the second sheet (30) by an axially extending join (28) at a position spaced from the end (14A) of the second sheet (14). The end (30A) of the additional sheet (30) is joined to the end (14A) of the second sheet (14) by an axially extending join (28). The additional sheet (30) is thicker and wider than the first and second sheets (12, 14) such that at least one of the hot and cold edges (30C, 30D) of the additional sheet (30) extend beyond the hot and cold edges of the first and second sheets (12, 14). The additional sheet (30) is wound around the first and second sheets (12, 14) about the axis (X) and the end (30B) of the additional sheet (30) is joined to the additional sheet (30) by an axially extending join (34).