Today I was tired enough to be incapable of working effectively, so it gave me a chance to get the steam knob off, as prompted by AM.
I'm not entirely certain how to measure a thread, but from edge to edge at the widest point of the thread, it looks to be 8mm on the nose.
The depth of the screw appeared to be 18mm. It looked straight by eye, but these photos have me doubting that!
Here are the shots:
http://imgur.com/tFn6S
http://imgur.com/GoNNN
http://imgur.com/hU6er