I'm not sure about an Astra but I suspect most DPF systems will operate in a similar manner and you should just drive normally on the motorway for the car to do an active regen. A common misconception is to boot it at high revs, but this is bad because you want reduce turbo boost so that the hot exhaust gases are not transmitted back to the turbo. The car will adjust the fueling automatically (among other things) in order to increase the temperatures to the necessary values so all you need to do is let the car do its thing. But, if there is a fault with your car then depending on what the fault is then it may not do an active regen.
Also, my car needs over 1/4 of a tank of fuel and I must be travelling over 45mph.
The one thing they really needed to do with all cars that have a DPF is some sort of warning that a regen is in process, because if you turn the car off before it has completed the regen you end up diluting the engine oil with fuel and robbing valuable lubrication properties. There's tell-tale signs such as the engine note changes, fuel economy plummets etc but not always obvious.
Try looking up icarsoft devices, they can sometimes have an option to show you the soot levels and also allow you to start your own active regen. Very handy.