I had this problem and followed Michael Dranfield's Youtube video (https://www.youtube.com/watch?v=bJ8HSLlOwA8) which worked brilliantly. The essence is that you use a freeze spray (plumbers or seized thread breaker, not the physiotherapist ones) to cool the NAND chip (IC1302 in my case) to about -35C (according to my IR thermometer). Once this is done, the TV will stay on and the software can be upgraded either via USB (which didn't work for me but I'm not sure why) or over the internet. I don't know why this works but it worked for me. The spray cost 5GBP and the alternative was sending off the motherboard (probably for the same fix) for 45GBP via ebay ads.