I have had another reply from Lexus customer care, this is the relavent section:
"I appreciate the issue you are experiencing is a frustrating matter. The issue related to the personal hotspot from Apple smartphones has been raised earlier this year with their iOS updates which has caused users passwords to not be captured. I understand you have tried and tested other systems/devices which work well with your smartphone, we are unable to comment on how they work in comparison.
I can reassure you Lexus are doing their utmost to improve the systems and software to ensure compatibility can met with the various smartphones that are available, in some instances there will be changes in a smartphone operating system which may alter performance and affect performances which is out of our control."
This implies the problem lies with Apple, but others on here, most recently Blue-Agila, are clear that Android phones are affected too.
I appreciate the solution involving a dongle, but this requires another piece of kit, and a different sim card = more expense. The system is supposed to work with a mobile phone, it does not. is there anyone else experiencing this problem?