I went to the dealer today. Checked it over and said they'd had several people in with the same and similar issues, but only Lexus models, not Toyota. They believe the last "over the air " update from Lexus has somehow corrupted the Multimedia system - but not affecting every vehicle and not always the same fault - some had Navigation issues as an example. It's been escalated to Lexus to release a fix and they have promised to ring me once a fix is ready and either have me back in to install it, or advise if it can be done over the air.
Will update as and when I get it resolved.