It might not be theft, reason I think this Is because if you fi to a petrol station, fill up and then go to the cashier with a bag of crisps and give them 70p, if this goes unchallenged its not theft - you have made a reasonable attempt of payment.
Give over. That's not at all reasonable. You're not telling them that you've picked up any fuel and you know what you're doing, ergo it is theft.