Hi Charlie,
I had similar problem LS400 2000. Had it seen to in3 different countries with no fault found. Last resort, Hatfield Lexus at £150/h. It turned out to be a wheel bearing and its housing at the cost of £1400!!!The garage that replaced dicks and pads replaced the discks fo r free and -voila- problem gone. I wish you luck with yours!
Best, Milic