My HUD failed 3 times in successive drives. I checked the display settings in iDrive and the box was ticked each time. I unticked it then ticked the box again but the HUD still did not work.
On the 4th occasion, the HUD was still not working but I noticed that the box in the display settings for the HUD was not ticked. Ticked it and the HUD started working again. Still OK after a few more drives. Very strange.