A well comprising an inner well casing, an outer well casing, a clamping arrangement and a seal for sealing an annulus in the well, the annulus being formed between an outer surface of the inner well casing and the outer well casing wherein: the inner well casing comprises: a central longitudinal axis which extends along a longitudinal axis of the inner well casing; and an outer clamping surface which is arranged to be secured and clamped in the outer well casing, the outer clamping surface being located at an initial radial distance from the central longitudinal axis of the inner well casing; the clamping arrangement comprises: a compression collar; and a compression ring wherein the compression ring and the compression collar have oppositely directed axially tapered annular surfaces so that relative axial movement between the compression collar and the compression ring, towards each other, reduces an internal diameter of the compression collar to distort the outer well casing inwards creating a clamping force which grips the inner well casing; characterised in that the seal comprises a flexible metal sealing lip which projects radially outwardly and away from the central longitudinal axis of the inner well casing, from a support surface of the inner well casing and wherein the support surface is of a reduced diameter relative to the diameter of the clamping surface of the inner well casing, the metal sealing lip having a distal sealing surface for sealing against an inner surface of the outer well casing; and wherein, the compression ring is arranged to exert sufficient radial force to distort the compression collar and the outer well casing inwardly to flex the sealing lip; and the sealing lip extends towards the side of the annulus to be sealed and a projection angle (θ) is defined between a pressure containing surface of the sealing lip and a support surface of the inner well casing, and the projection angle (θ) of the sealing lip is arranged to decrease as the sealing lip is flexed inwardly by the clamping force and wherein the projection angle (θ) is urged towards a greater projection angle (θ) as the sealing lip is energised by pressure in the annulus acting on the pressure containing surface which is arranged to urge the sealing surface towards the inner surface of the outer well casing. the inner well casing comprises: a central longitudinal axis which extends along a longitudinal axis of the inner well casing; and an outer clamping surface which is arranged to be secured and clamped in the outer well casing, the outer clamping surface being located at an initial radial distance from the central longitudinal axis of the inner well casing; a central longitudinal axis which extends along a longitudinal axis of the inner well casing; and an outer clamping surface which is arranged to be secured and clamped in the outer well casing, the outer clamping surface being located at an initial radial distance from the central longitudinal axis of the inner well casing; the clamping arrangement comprises: a compression collar; and a compression ring a compression collar; and a compression ring wherein the compression ring and the compression collar have oppositely directed axially tapered annular surfaces so that relative axial movement between the compression collar and the compression ring, towards each other, reduces an internal diameter of the compression collar to distort the outer well casing inwards creating a clamping force which grips the inner well casing; characterised in that the seal comprises a flexible metal sealing lip which projects radially outwardly and away from the central longitudinal axis of the inner well casing, from a support surface of the inner well casing and wherein the support surface is of a reduced diameter relative to the diameter of the clamping surface of the inner well casing, the metal sealing lip having a distal sealing surface for sealing against an inner surface of the outer well casing; and wherein, the compression ring is arranged to exert sufficient radial force to distort the compression collar and the outer well casing inwardly to flex the sealing lip; and the sealing lip extends towards the side of the annulus to be sealed and a projection angle (θ) is defined between a pressure containing surface of the sealing lip and a support surface of the inner well casing, and the projection angle (θ) of the sealing lip is arranged to decrease as the sealing lip is flexed inwardly by the clamping force and wherein the projection angle (θ) is urged towards a greater projection angle (θ) as the sealing lip is energised by pressure in the annulus acting on the pressure containing surface which is arranged to urge the sealing surface towards the inner surface of the outer well casing.