Company claims late payment due to unreceived invoices

Hi there, a company that I am dealing with is claiming that they didn't receive my invoices and therefore won't pay late payment charges. Am I right in thinking that as they were aware of both my prices and the exact dates that I worked in our contract, that I can apply the rule of the relevant period of time of 30 days "on which the obligation of the supplier to which the debt relates is performed"? This would actually leave them worse off as my invoices were sent to them a few weeks after working.

Thanks!