I was unfortunate enough to witness this accident at Osea island in Essex. I was on a boat, anchored only a couple of hundred yards away and I will never forget watching it spin in to the ground. The cause, CO poisoning. Decent CO detectors are not expensive i find it hard to believe that people wouldn't have one in their aircraft...
http://www.aaib.gov.uk/cms_resources...pdf_507786.pdf