Well the noise has been fixed.
One of the locating dowels fro the clutch pressure plate from the flywheel came loose, and has been loose in the bell housing, and under braking or hard cornering is coming into contact with the clutch cover / pressure plate and making th noise.
All sorted by stripping the gearbox out, re fixing the dowel, new clutch, pressure palte and bearing, and all back together.
If only BMW had listened four months ago, I wouldn't have had to do this myself.
Oh well, all sorted, and a pleasure to drive once again.