Easy answer would be to multiply your original rates times the number of usages.
If they balk at this, your reply should be something like "If you disclosed the full usage of the material at the time of the original contract, we could have worked out a deal for the costs. But as you've taken the material and shown it beyond the scope of the original contract, we're charging you fair market rate for each showing."
For our situation, I think that response may be a little hard core! They are a fantastic client and this is the only time in 3 years that I have had something to complain about.
I guess what I am really asking is what should I be doing at the quoting stage - if the ad is going to be shown internationally, how do you quote usage rights? Is there an industry standard, or do you just come to an agreement with your client on a per case basis?
Ignorance is a poor defence, so I am just trying to make sure I don't get into this situation again.