A resilient unit has strings of individual coil springs (not shown) inside pockets (120) are arranged in an array. The strings together form a spring unit and are each secured to a common cover sheet S, which comprises a non- woven elastic fabric. It is welded ultrasonically or thermally whilst under tension in one or both of its major dimensions - i.e. in-plane - to the pockets (120), the material of which is substantially inelastic and non- woven. In this example the welds, labelled W, are located substantially centrally with respect to the generally circular end surfaces of the pockets.