Brompton headset problem?

Epoch Posts: 23
edited November 2007 in Road general
After much debate i hav brough a brompton from my lbs

However it has developed a annoying problem.
When i use the front brake hard there is a loud click that jolts the handle bars a sort of notch sensation

After much looking it isnt the clamp and is not the handle bar clamp fitting either so im presuming its the head set?

I brough it back had it tightened and it seemed to solve it for a few miles then the same problem

The bike is only 3 weeks old can anyone shed any light on wht it might be?