Thanks for the advice on welding/brazing and ebay.
The main thing is that the freaking bike didn't break while I was riding it. And I am very grateful about with my general state of good health.
Regarding ebay, I didn't know that I could still ask for a refund after giving positive feedback, so I will look into that. I have just emailed the seller just to make the point that I am ingood health but bad things could have happend.
Failing this I might ask the advice of local welders but I am now unsure of the reliability of the repaired frame. As tradtimbo pointed out this could be a crack along the welded seam of cheap(er) chromoly. The On-one website is ambiguous about this:
"We use different profiles of DN6 depending on the frame. The Inbred uses the lightest grade, the Gimp a custom drawn grade for dirt jumping, and the Runt uses a super-strong straight-gauge format for BMX abuse."
When I asked On-one "is the tubbing seamed or drawn?" I still got an ambiguous answer:
> the Inbreds are now custom DN6 double
> butted 4130 chromoly steel tubing
> that part of the website is really old.. the Gimp and Runt
> are dead....
Mine is an older (2005?) bike with the sliding dropouts, but I don't even know if the new ones are drawn or seamed. Is a Kona Unit 2-9 made from drarwn chromoly tubbing or are cheaper frames all the same?