The present invention provides clamp head comprising an arched body having upper and lower faces, a vertex and opposing unequal length arms. The body is further provided with a through aperture for a bolt or like threaded fastener which extends between the upper and lower faces in the region of the vertex. The aperture in the upper face is oval and tapers outwardly through the body in the direction of the lower face. The arms comprise a reaction arm having at its distal end a reaction face and a clamping arm having at its distal end a clamping face, the reaction and clamping faces being provided with respective formations. The formations of the reaction arm extend in a direction which is substantially transverse to the direction of the formations of the clamping arm.