Wish I'd done this a long time ago. I bought a 5Kg bag of "crumb" from a foam shop for £13 and with a little nifty stuffing and restitching have revived all our sagging back and arm cushions in less than an hour. It's taken years off our ancient sofas! together with our sofa savers, we'll get another few years from our pair yet. (Scarily satisfying!)
Well done. I've always been a bit dubious about those sofa savers, do they really work then? Our sofa is knackered getting to the point where there's no support under the seats but its nice and I dont want to change it just yet.