I have an RX400H and had both front and rear sensors fitted for £470 all inclusive. Didn't get a lexus dealer to do it though but some guy who used to work for Lexus and was actually recommended to me by a Lexus salesman I know very well
I have an RX400H
Not a diesel but also noted difference in fuel economy between places. On BP I get around 33 mpg. On Tesco petrol do better at between 35 and 36 mpg.