Vauxhall Astra intermittent engine problem

I have a 2015 Vauxhall Astra (petrol) with a strange intermittent problem, hoping someone might have experienced something similar.

The car lost power and stalled after driving perfectly normally. It would not re-start.
After about 2 hours, it started and drove normally for about 40 miles over 2 days – then did exactly the same thing again.

Mechanic found no codes, but suspected various sensors and changed them all.

He has left the car running (idling) for up to 5 hours. Sometimes it then drives normally, others it stops when you try to drive it. So it seems like it might be temperature related.

In response to a similar question on this site, an answer suggested the fuel pump binding when at temperature. This is possible, but I find it unlikely, only because I saw the petrol gushing from the pipe when turning over and the car would not start.

Mechanic is currently suggesting ECU fault – which seems unlikely to me, and will also be an expensive replacement.

Any suggestions or things that are worth considering please?