IMHO, I don't think any refund is due. Maybe as a gesture you can offer 50 Euros next 1 hour booking (assuming you refund 100 Euros), but then you would do that only because you are being nice and not because you are obliged. If this is too much of an hassle you can offer partial refund, again as a gesture.
If a guy stays overtime (even for 10 minutes), do you demand extra money? Its a two way street.