Got the bike back!
Dropped it off to Balfes bikes in Waterloo, all sorted under warranty without any drama.
Got a call a few days after dropping it off to say Brompton would honour the frame warranty. Bought it second hand (in almost new condition), previous owner had emailed Brompton to change ownership on my behalf.
Only problem was there wasn't a blue triangle available for a while, so black it was.
Was probably due to replace the triangle hinge anyway, as it'd developed a little play, which I can't detect now.
Overall pretty happy with the service from both Balfes and Brompton.
by Mick_kerr