hate intermittent faults like this, as 99% of the time the dealer won't be able to find them.

Sounds like it could be either software or hardware intermittent issue.
Most cars these days have some software oddities, which are usually fixed by doing a reset, either infotainment system or the complete car (12v battery disconnect). Could be the actual HUD connector not properly seated, but I doubt that’s easy to get to to check and reseat.
Assume MB has an infotainment reset, so try that first.

 

Video it if it happens next time?

 

I hate intermittent faults like this, as 99% of the time the dealer won't be able to find them.

Smells like a poor connection to the HUD, though there's no proper way to diagnose that, not even to guess whether it's +12V power, earth/chassis, or data. The dealer might be able to read a stored fault code, though my guess is that all it will show is that the car's main components lost the data connection to the HUD unit. It's worth trying though, in case there's more detail than that stored. If it's a software fault the diagnostic code ought to be more informative.

 

Seems to be OK again this morning, been into town and back, three or four starts, and no sign of the problem. I do hate intermittent faults like this!

@Todor Thanks, brilliant idea to video it, especially as the error message on the right hand screen is persistent and has to be manually cancelled each time it pops up. At least having a record would prove the dealership that the fault is real.

 

I suspect MBUX loading issues and that it has now reset. We have the same with the internet connection for musique en ligne it does not always load and connect after a short stop (popping into the boulangerie on the way home sort of stop). Something that has happened in all the cars with internet in the car I have had previously.
Specifically in 9 yrs in 4 different models of having Mercedes HUD it has proven to be pretty stable but not without the odd glitch. The c class sometimes would come back as lines only especially in hot weather, the e class was less prone to that cannot really recall an issue with the EQC and think once the HUD failed to load in the EQE but no error message. In all cases normal services was resumed after a longer downtime.
We do have some oddities regularly occurring with the digital lights and profiles with settings being lost and I am increasingly thinking there are some OTA being sent as other things are looking a bit different but the car is so complex now half the time I think it is me🙄🤣 however after any software work by the garage the settings in profiles seems all over the show and one has to go through it and check it all again.
The EQE SUV was dogged with some early issues of MBUX throwing a hissy fit it would throw up all sorts of error messages and some of that was 12v low battery related (now cured by an update) but I think there were also other software gremlins, my car was delivered without the ambient lighting working properly for example which was a simple software update or reboot well I say simple it took 2 days due to server congestion.
Now I do have the advantage of having a garage that knows it’s stuff and whose technicians and service manage have been trained for the EQ range and indeed all models in detail which do differ unlike it seems the average U.K. operation and I would also say the USA.
So if the problem continues in any discussion with the garage ask them about a software fault, I would say for mine it is the first thing they talk about and for the ambient lighting they were on it straight away also our experience of error messages, we have had 2 door handles and there again they identified the likely cause before we even took it in (that was a physical problem and I suspect a bad batch probably down to a poor OEM supplier as has happened to USA built models in the past).
I believe there are ways to reset the system involving locking the car and moving the keys well out of range and leaving everything for around 20mins then when the car comes back to life it does a required reboot which is enough to reset issues.