Apparatus and method for filling and sealing a chamber with fluid at a predetermined pressure in a well bore. In a downhole assembly (10) comprising a tubular body (14) having a cylindrical throughbore (18) and a chamber (16) to be filled on an outer surface (26) of the body, a fill mechanism is provided to control fluid flow between the throughbore and the chamber. The fill mechanism includes a sliding seal (72) arrangement at the outer surface which is operated by the fluid flow in the throughbore to allow fluid flow into the chamber and then seal the chamber. Embodiments are described where the chamber is between a morphable sleeve (64) and the outer surface of the tubular, filling of the chamber morphs the sleeve and sealing the chamber at a predetermined fluid pressure secures the tubular within a borehole, creates an annular seal across an annulus or centralizes the tubing within a wellbore.