You sure it's not just that both rear lights have blown ? Or the fuse ? Is the bulb failure light warning lit on the dash ?
No, the bulbs and fuse are OK. I have spent some time on it today and in the end popped into a Lexus dealer. they confirmed that the lamp failure unit was the reason, it will cost over £200 for a replacement which will take at least 10 days to arrive!!
By the way, the sensor is on the passenger side of the boot, hidden behind a strut.
Many thanks for your help, it was your wiring diagram, pages 210 & 211 which helped me to find the problem, which is very unusual, according to the lexus mechanic who helped me