I have removed many warps over the years using this method, you need a heat source, preferably gas ring and the biggest heaviest book you can lay yer hands on ( must be a hardback). open the book halfway, place the warp on the disc over the gas flame ,(about 18 inches above it), slowly ( and I mean slowly) bring the disc down towards the flame, if you do this at the right speed it will slowly start to droop, at this piont place it as quick as possible in the book, slam the book closed. I have straightned a good few discs like this, the good thing about it is if you dont get it right first time you can have another go at it, provided you dont let it get too hot first time round.
copywrite: the jules record straightening method