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All Vauxhall petrol caps open without the key nowadays as the flap itself locks when you lock the car.
All you need to do is once the flap is open, is turn the cap anti-clockwise like unscrewing a bottle top. They can sometimes be tight but it will go with a bit force. They usually onlt click when you are turning them clockwise after they have tightened up enough so that you don't break it.
If it is clicking when turning anti clockwise try putting a bit of inward pressure while turning it as the last person may have over tightened it slightly.
If it still won't go then you are going to have to taker it back to the garage tomorrow and they will have to remove it for you as the cap may be broken but try all this first though.
I hope this helps you in this matter.
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