<p>A composite plastics structure in the form of two parts of an inlet manifold housing (11) is formed by joining a first component (16) to a second component (17) at a pair of abutting non-planar joint faces (27, 28) indicated by a joint line (18). Lands (37, 38, 39 and 40) on the joint faces define gaps which are substantially eliminated when tongue (29) on the first component is welded to an abutment surface (33) on the second component by vibrating one component relative to the other. Where the joint face is inclined to a clamping force the projecting distance of the tongue is reduced. The first component is more flexible than the second component to permit the joint faces to conform under the clamping load.</p>
