I am looking for advise from people who have done this in the past.
I have ridden it 7 times on fixed, once (the first time) on gears
What gear did you ride?
68-70"
Would you do it again?
Yes, like a shot
I am a complete nutter for even thinking about it?
Hard to say. It was my first 600 on fixed, but I'd done a couple of 400s and several hilly 300s and I was pretty confident.
How far have you done on fixed in one go ? If you've not yet done a 300 or 400 with no issues, I'd be cautious about going straight to a 600. It's not so much the terrain as the cumulative discomfort. If you haven't got the fixed miles in, it'll hurt.
Edit: it's not an overly hard 600 really. The Denmead 600, for example, is a lot harder I think. The question shouldn't be "Bryan Chapman on fixed ?" but "a 600 on fixed ?"