So, a brief update. Took the car in to the dealership for them to try the 'hard reset' to try and reinstall the sat nav function.
After 2hrs wait the service chap came out to say the reset didn't work and Techstream is reporting the Sat Nav module is disconnected.....
He explained they would have to remove the multimedia unit to check the connections and this would take a good few hours and he would arrange a courtesy car. Fine I said, no problems.
He then started to look rather apologetic and explained that the 2hrs diagnostics wasn't covered under the warranty but he could try and get a 50% reduction as goodwill. I thanked him for the offer but explained the car is just 2yrs old, the Sat Nav was functioning normally and now isn't so I don't expect to pay for them to discover what must surely be a manufacturing problem (loose connector, broken wire, broken nav module) and he should have a word with his manager.
Two mins later he returned to say everything was covered and hoped it would be sorted to my satisfaction and that they could restore my confidence in Lexus. I think he got a rocket from his manager!
So, a rather nice NX300h is sitting on my driveway and I await the phone call as to what the problem was.