UPDATE (for what it's worth)
My Lexus technicians here in the UK have said they now know of the problem. 50% of customers have this problem and 50% had the problem which was resolved by an update (I am cynical about that info).
Either way, I was asked to go in to the tech's workshop last week where they plugged in their diagnostic laptop, took a load of data from the system and then videoed the error in progress (which, luckily, didn't let me down by being the random time it worked - which does happen very occasionally!)
I was told that Lexus/Toyota are aware of it. They outsourced their software contract to a new company from August last year and the more cases they build up, the sooner it can be resolved. They suggested those who have cars with the pre-August software should be ok but those, like me, who are post-August are reporting the same issues we all have.
I am hoping for an answer some time soon - in a few weeks, I guess.