A subterranean assembly which comprises: (a) a cylindrical assembly (11, 12) which includes inner and outer concentric walls (14, 15) which define an annular space between them, and (b) an annular seal assembly which includes at least one chevron seal which is made of metal or metal alloy, the chevron seal including an annular central body and two annular wings (27b; 27c) extending radially and axially from the central body, one of the wings extending radially inwardly from the body to a distal annular end for sealing engagement with the cylindrical inner wall of the cylindrical assembly, the other wing extending radially outwardly from the body to another distal annular end and being arranged for sealing engagement with the cylindrical outer wall of the cylindrical assembly (11, 12), characterised in that: (i) the radial extent of the chevron seal measured between the distal annular ends of the inner and outer wings (27b; 27c) is greater than the radial extent of the said annular space between the inner and outer walls (14, 15) of the cylindrical assembly (11, 12), when the seal assembly is outside the annular space, and (ii) the inner and outer wings (27b; 27c) are deflected towards one another and resiliently spring loaded against the inner and outer walls (14, 15) respectively when the seal assembly is installed in the annular space. (a) a cylindrical assembly (11, 12) which includes inner and outer concentric walls (14, 15) which define an annular space between them, and (b) an annular seal assembly which includes at least one chevron seal which is made of metal or metal alloy, the chevron seal including an annular central body and two annular wings (27b; 27c) extending radially and axially from the central body, one of the wings extending radially inwardly from the body to a distal annular end for sealing engagement with the cylindrical inner wall of the cylindrical assembly, the other wing extending radially outwardly from the body to another distal annular end and being arranged for sealing engagement with the cylindrical outer wall of the cylindrical assembly (11, 12), characterised in that: (i) the radial extent of the chevron seal measured between the distal annular ends of the inner and outer wings (27b; 27c) is greater than the radial extent of the said annular space between the inner and outer walls (14, 15) of the cylindrical assembly (11, 12), when the seal assembly is outside the annular space, and (ii) the inner and outer wings (27b; 27c) are deflected towards one another and resiliently spring loaded against the inner and outer walls (14, 15) respectively when the seal assembly is installed in the annular space. (i) the radial extent of the chevron seal measured between the distal annular ends of the inner and outer wings (27b; 27c) is greater than the radial extent of the said annular space between the inner and outer walls (14, 15) of the cylindrical assembly (11, 12), when the seal assembly is outside the annular space, and (ii) the inner and outer wings (27b; 27c) are deflected towards one another and resiliently spring loaded against the inner and outer walls (14, 15) respectively when the seal assembly is installed in the annular space.