So far one of you has told me:
"I rode past it on my bike just as ambulances arrived on scene; rider was down but seemingly conscious and stable".
Use the comments for more updates as and when in the normal way...
1030 UPDATE - The Police have confirmed to me that the rider is a woman and that she is basically OK. Here's what a spokeswoman had to say:
Police were called by the London Ambulance Service at 09:28hrs to reports of a collision along the Uxbridge Road j/w Southern Roundabout, W12.
Female being treated for shock.