Downhole apparatus and methods

A method of locating bore-lining tubing, such as a liner (120), in a drilled bore (106) comprises selecting a buoyant material, such as air (138), having a density lower than the density of an ambient fluid, such as well fluid (180, 182). The buoyant material (138) and an inner tubing (140) are located within the bore-lining tubing (120) with the inner tubing (140) extending from a distal end of the bore-lining tubing to a proximal end of the bore-lining tubing. The inner tubing (140) is sealed to the distal end of the bore-lining tubing (120) and to a portion of the bore-lining tubing (120) spaced from the distal end to define an inner annulus (152) between the inner tubing (140) and the bore-lining tubing (120). A volume of the buoyant material (138) is retained within the inner annulus (152). An assembly (168) comprising the inner tubing (140) and the bore-lining tubing (120) and containing the volume of buoyant material (138) is run into a drilled bore (106). Fluid (126a) may be flowed through the inner tubing (140) and into an outer annulus (124) surrounding the bore-lining tubing (120).