Thank you Steve
The fault came on independently of any other work. In fact the car has needed nothing than regular servicing and tyres!
I have asked the dealer to contact Lexus and ask for a goodwill gesture as I suspect it may be an issue Lexus needs to examine more closely. If any more of this model turns up with the same issue, it could be a design flaw and with the implication of air bags not deploying in an accident, might require a recall.