Hello and Welcome to justanswer...
Your post has been moved to the Uk section now.
The Astra that you have suffers with electrical problems / faults with the electrical section on the very rear of the ignition barrel. Sometimes you even see smoke coming up from the ignition area.
This controls all the power from the ignition and it would seem evident from what you describe that you have this problem.
The part is fairly easily replaced and a common part that the Vauxhall dealers keep in stock usually because of that fact.
To replace the said part - you need to remove the plastic cowling from around the ignition and once you have the new part in your hand - you will see exactly how the old one is removed and the new one fitted.
As a further thought --to be sure - check all the wiring under the bonnet - and especially the connections to and from the battery - Check too the earth strap - from the battery to the engine and the engine to the body.
Note - a common failure on these too -is the braided earth strap that connects the engine to the body. You can find this by looking down on the battery side of the engine bay - look at the very end of the gear box - and you will see the earth strap that connects on the end of the gearbox and then to the body. The braided strap usually falls to bits and breaks. If you find that strap to be at fault - that will be your fault. Fit a new engine to body earth strap.
I Sincerely XXXXX XXXXX have helped you - Please ask me if you need more help...
Kind Rgds,.... MIKE
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All the best... Kind Rgds... MIKE