Incomplete journeys
You can apply for a refund by selecting a journey and providing the missing information. You can make 3 application(s) per calendar month from the list below. You must apply within 8 weeks of the journey.
Your refund will usually be the difference between the maximum fare charged and the advertised fare for the journey made.
I'm not in London, sorry hadn't made that clear. In London most stations have barriers? I couldn't forget so easily in that case. Maybe I should relocate. Are the streets really paved with gold?
Just make an account for your card and apply for the refunds
https://oyster.tfl.gov.uk/