Engine damage from Diesel Particulate Filter (DPF) cleaning process
Modern diesel Astras have diesel particulate filters (DPF) fitted.
From Vauxhall:
"The diesel particulate filter system in your Vauxhall filters harmful soot particles out of exhaust gases. It includes a self-cleaning function that runs automatically during driving when the filter needs cleaning. This can take up to 25 minutes and fuel consumption may be higher during this time. You may also notice an emission of odours and smoke and an increase in heat, which is a normal part of the process."
If for some reason this process repeatedly fails to complete then you could end up with engine damage through contamination of the oil system with fuel. This will lead to an increase in oil level which will have consequences for the engine.
What to do:
The solution to this problem is to make sure that the cleaning process completes on your car, running it for 25 minutes is enough and the process is quicker at higher engine speeds.
If you're looking to buy a diesel Astra you should make sure that problems connected to the DPF have been checked and the car has a clean bill of health before parting with your money.