Thanks for the ideas, I can at the moment hold the Rivnut still because I can hold the bolt that's still suck in it ! I would be really nervous of trying to grind it out as I would certainly make a mess of the frame. My idea is to try and hold the nut head and drill down the shaft of the nut with progressively bigger drills until it's sufficiently week and then try to crush the flange of the Rivnut until small enough to push into the frame. Once done then fitting a new Rivnut seems easy enough with the correct tool. I would probably just buy the tool use it and then stick it on
ebay for the next person who cross threads their bottle cage ! If I could find a way to get the bolt out easily the process would be so much easier as the rivut would quite easily drill or crush but there is so little meat on the flange of the Rivnut you cannot get hold of it.