described with reference to fig8 to 14 where the same features are present in the second embodiment which are present in the first embodiment the same reference numbers have been used the design of the second embodiment is the same as that for the first embodiment except for the design of three features namely 1 the design of the seal 40 has been altered 2 the provision of an additional aperture to the two apertures 36 and 3 a new feature of a pressure release mechanism for the recess 38 has been added referring to fig1 12 and 14 the opening 80 of the recess 38 is planar ie the periphery of the opening 80 locates in a flat plane 82 the seal comprises a base 84 which locates in a groove 42 formed in the wall 52 of the skirt 14 the base 84 of the seal is held within the grove 42 via cover 90 which is held in place on the housing screws not shown a flange 86 is attached to the base 84 which extends from the base 84 in a direction which is an oblique or small angle 88 relative to the plane 82 the flange 86 extends in a direction parallel to the plane 82 in the direction of arrows m away from the opening 80 the flange 86 extends in a direction perpendicular to the plane 82 in the directions of arrows n away from the opening 80 the outer perimeter of the flange 86 is supported by a secondary wall 92 formed on the shirt 14 of the housing 10 secondly the additional aperture 94 and its inter connection with the recess will now be described referring to fig1.