What to do if a client does a runner without payment?

Hi all, I need some design advice!

What do you do when you think a client is about to do a runner without paying you?

My cliemt has not paid my last two invoices and keeps asking for the working files to 'check my work' before he pays me.
He's also gone quiet on the email front when I mentioned the terms of the contract that files will be exchanged once payment has cleared.
The contract I have with him is a long term, per hour contract, that is authorised to 40 hours a week so I didn't ask for a 50% down payment at the start.
It's also a bit more difficult as he is my first international client (outside of Europe) so I don't know where I stand in seeking payment across international borders.
I've sent across mockups of the logo, branding and support collateral for campaigns he has coming up so Im worried that he will take these and use them / amend then himself as he told me he had downloaded the Adobe suite to 'check my working'.

Thanks in advance,
From a worried designer.