outer surface of the upper end of the upper assembly 136 this cover also acts as a secondary metal to metal seal against the environment spaced substantially equidistant around the tree 105 and connected to the conductor 124 via a guidebase 184 are a plurality of guide rods 185 the upper ends of which are engageable with lower ends of an equal number of mating rods not shown retained in association with a further assembly a further assembly not shown may be a supplementary christmas tree provided for safety reasons in the event of failure of the tree 105 the further assembly may be supported by a semi submersible rig or floating platform a further valve 190 is provided within the tube 125 a lower end of this valve 190 being connected to a further conduit 195 extending into the borehole through the well head tube 125 and tubing hanger 126 while an upper end of the valve 190 is connected to the upper assembly 136 communicating with a bore 196 the further valve 190 is preferably also a ball valve the function of the further valve 190 and conduit 195 is to act as an annulus pressure control valve as is known in the art the embodiment of the invention described hereinbefore is given by way of example only and is not intended to limit the scope of the invention in any way it should be appreciated that the disclosed invention provides a particularly simple and easily constructed solution to problems experienced in this art and in particular to problems experienced within the realm of seat supported ball valves it.