Hmmmm....a tricky one. Not here to judge, just taking an objective view.
1.) If your girlfriend said the correct and true pump number and the cashier charged you the wrong amount then cashiers fault.
2.) If your girlfriend gave the wrong pump number (whether knowingly or unknowingly) it is your girlfriends fault.
3.) If the cashier gave a wrong pump number and your girlfriend said "yes" then this is a bit more messy yet technically still your girlfriends fault as she has lied (knowingly or unknowingly) to the cashier. Why? Because the cashier is operating on his/her best knowledge that the pump you used is the one that they think you used. The cashier tries to confirm their judgement by asking your girlfriend. Your girlfriend agrees with this judgement and unknowingly tells a lie.
I think technically you owe the petrol station some money as you have taken petrol (albeit approx 7-8 litres) that you have not paid for. In a criminal court of law, this is classed as theft.
BUT....It's a moral dilemma as at the moment you seem to have "got away with it" and I doubt that the petrol station will chase you for it. But they are entitled to and they have the means to. If a policeman knocks on the door, you can claim it was honest mistake and that of course you will pay £8.50 back to the garage. But, technically you now cannot 'honestly' claim that you 'honestly' did not know and were unaware of the mistake.....because you do know. Also, are you prepared to live like that for the next few weeks, waiting for a knock on the door?
Finally, I wonder if they had overcharged you for fuel, say about £40 total OR if you had paid for someone elses fuel and it was more than the cost of your fuel, would go back and ask for a refund?
Phew, that was a bit heavy!