Hello Brian
There is a five part (10 minutes each) series on Warburton on You Tube. Here is a link to part 1:
http://www.youtube.com/watch?v=wbna5nM6NHI
The conclusion is that he was downed by flak although I see no reason why it could not have been a combination of ME-262 and flak. He was accustomed to fly much lower than the photo recce pilots in Northern Europe and that may have contributed to his loss. His mental state was not the greatest according to some guys checking him out for the mission that day. My father was a pilot with the 7th PRG at the time and enjoyed talking with Warby during his less than 2 weeks with the group. They tried to teach him American baseball and he was incapable of catching the ball and took it on the forehead a number of times to the amusement of everyone present (Warby included).
All the best
Scott Blyth