heres the situation, I found one of my images being used by a commercial website. I have invoiced them for the usage and so far they have failed to make payment.

I have an email from them apologising for using my image so there is no dispute that they were using it (they have now removed it). However ever since I invoiced them they have failed to answer my emails. If they dont pay the invoice what process should I follow to obtain payment?
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Richard How FIAP MAPM
How Associates - Programme Management Consultants
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