Just push the seat down - I know it's a pain in the butt but it does come out. Do what Ken said and push really hard on the left (or right) seat pad (but not the centre seat area) in approx the area of the middle and towards the back (basically where your backside goes!!) and the clip will release - eventually!!
If you are still strugling after those sugestions. Here is what helped me in Astra sport toures K 2019. When i pulled out front side of seats on the left and right side, i still was not able to remove them. The key was to remove covers on 4 ISOFIXes. After that i was able to lift seats little bit higher and see what is holding them. There were together 3 hooks. On each side and in the middle. If you will see it, you will know how to remove them.