Help - engine cuts out when lights are turned on!

As a starting point I would be looking closely at every earth connection I could find.

Si

The one between the alternator & the engine & the one between the engine & the chassis being favourites

Thanks. I'll check those if I can see and get to them easily enough.

Is this a common problem on these Astras?

Earth problems are a common fault on a great many vehicles as the connections corrode over time. It's unlikely to be corrosion on something as new as yours though so maybe the alternator's been taken off at some point and not put back on properly? Check the battery connections are clean and tight too.

Si

Sounds like a bad connection somewhere, causing the ECU to shutdown (hence the lack of codes). A quick visual check of the earths is a good starting point, along with any other visible connections (with the engine running and lights on, give the connections a wiggle)

After that, it's a case of finding out if the ECU is loosing it's earth, or battery supply, and then isolating sections of wiring/relays until you find the problem.