if someone is looking for an image that cost them nothing, then no matter if they've used yours or not, you've not lost a sale as there was never going to be one for you as the user wanted an image that didn't cost them anything. i'm not saying it's correct, but i'm saying the sale was never there to lose, so you can't claim that each time someone uses your image you've lost £200
Perhaps, but they have still used the image, if they wanted it free then choose another image....
Personally if it was the situation in the OP then (if they had bought a lot of images from Getty) I would appear like an honest mistake and should be cancelled with the payment of the fee the image cost should have been.