A seal assembly for sealing an annulus between an inner tubular member and an outer tubular member, the seal assembly comprising: an annular first seal which in use sealingly divides the annulus into first and second parts (16, 18) and which in cross section includes: a generally U-shaped first portion which comprises a radially inner first leg and a radially outer second leg, the first and second legs defining a first cavity; a generally U-shaped second portion which is located radially outwardly of the first portion and which comprises a radially inner third leg and a radially outer fourth leg, the third and fourth legs defining a second cavity; wherein the second and third legs (28, 32) are joined together between the first and fourth legs (26, 34) such that in use the first cavity faces the first part of the annulus and the second cavity faces the second part of the annulus; wherein in use the first leg engages the inner tubular member and pressure in the first part of the annulus urges the first leg into sealing engagement with the inner tubular member and forces the second leg radially outwardly to thereby urge the fourth leg into sealing engagement with the outer tubular member, and in use the fourth leg engages the outer tubular member and pressure in the second part of the annulus urges the fourth leg into sealing engagement with the outer tubular member and forces the third leg radially inwardly to thereby urge the first leg into sealing engagement with the inner tubular member; and wherein the seal assembly further comprises a first support ring which is positioned in the first cavity and in use is causing the first leg to remain engaged with the inner tubular member under the influence of pressure in the second part of the annulus, and a second support ring which is positioned in the second cavity and in use is causing the fourth leg to remain engaged with the outer tubular member under the influence of pressure in the first part of the annulus. an annular first seal which in use sealingly divides the annulus into first and second parts (16, 18) and which in cross section includes: a generally U-shaped first portion which comprises a radially inner first leg and a radially outer second leg, the first and second legs defining a first cavity; a generally U-shaped second portion which is located radially outwardly of the first portion and which comprises a radially inner third leg and a radially outer fourth leg, the third and fourth legs defining a second cavity; wherein the second and third legs (28, 32) are joined together between the first and fourth legs (26, 34) such that in use the first cavity faces the first part of the annulus and the second cavity faces the second part of the annulus; wherein in use the first leg engages the inner tubular member and pressure in the first part of the annulus urges the first leg into sealing engagement with the inner tubular member and forces the second leg radially outwardly to thereby urge the fourth leg into sealing engagement with the outer tubular member, and in use the fourth leg engages the outer tubular member and pressure in the second part of the annulus urges the fourth leg into sealing engagement with the outer tubular member and forces the third leg radially inwardly to thereby urge the first leg into sealing engagement with the inner tubular member; and a generally U-shaped first portion which comprises a radially inner first leg and a radially outer second leg, the first and second legs defining a first cavity; a generally U-shaped second portion which is located radially outwardly of the first portion and which comprises a radially inner third leg and a radially outer fourth leg, the third and fourth legs defining a second cavity; wherein the second and third legs (28, 32) are joined together between the first and fourth legs (26, 34) such that in use the first cavity faces the first part of the annulus and the second cavity faces the second part of the annulus; wherein in use the first leg engages the inner tubular member and pressure in the first part of the annulus urges the first leg into sealing engagement with the inner tubular member and forces the second leg radially outwardly to thereby urge the fourth leg into sealing engagement with the outer tubular member, and in use the fourth leg engages the outer tubular member and pressure in the second part of the annulus urges the fourth leg into sealing engagement with the outer tubular member and forces the third leg radially inwardly to thereby urge the first leg into sealing engagement with the inner tubular member; and wherein the seal assembly further comprises a first support ring which is positioned in the first cavity and in use is causing the first leg to remain engaged with the inner tubular member under the influence of pressure in the second part of the annulus, and a second support ring which is positioned in the second cavity and in use is causing the fourth leg to remain engaged with the outer tubular member under the influence of pressure in the first part of the annulus.