Does the Astra have an accelerator cable or is it an electronic throttle?
If its electronic, the throttle position switch can cause the engine to go into "limp mode" which sounds like what has happened to the car if the throttle butterfly is sticking due to carbon build up.. Remove the air intake tube and check for carbon build up on the throttlebutterfly and intake throat. Use car by cleaner to blast the accumulated muck out with the engine running at fast idle.
Even if it uses a cable, the valve can stick and cause problems so look at this area first. Some carbon could be very stubborn and will need to be carefully scraped out with a plastic scraper. Don't use a steel scraper or you could scratch the intake throat which is made of soft alloy.