I've just experienced exactly the same problem except I was stopped by friendly policeman who advised me to have it seen to immediately, had the same response from Lexus Teeside about possible igniter unit which would be covered by warranty.
They did not have either in stock as they said they had never experienced a failure so ordered one to be fitted next day, you've guessed it, it was the bulb which had failed: £212 plus labour.
To be fair they discounted this down to £197 including £22.50 labour which I paid grudgingly, but I was shocked and stunned when I returned home and did some research to find I can buy exactly the same item Philips D4S XenEco Xenon HID Bulb 42402 for £71 or a pair of upgraded Toshiba D4S Xenon HID Bulb 6000K Brilliant White UK for £70 from HIDs4U who are based in Southampton and offer next day delivery.