What helmet brand is it? Try the manufacturers website, for example, if you had a HJC, right on their website it says:
"Although your HJC Helmet is constructed with the best materials available to offer a long-lasting product, it will eventually need to be replaced. Immediately replace your helmet if:
• It has suffered an impact. Your helmet is only designed for ONE impact. An impact may fracture the outer shell or compress the impact absorbing liner. You may not be able to detect this damage. Any impact in a crash or a drop from as low as 4 feet is enough to damage your helmet."
http://www.hjchelmets.com/helmet_usage.htmA letter with the above in it (if yours is a HJC, and maybe even if it isn't) should do the trick. Basically, you have to make it clear to the insurance company that you aren't going away and are happy to spend time arguing with them over it, in a mature and professional manner
You could try this:
"You should not purchase a second-hand helmet, as it may have been involved in a crash or have experienced UV degradation, nor should you lend your helmet to others, as it can affect how the helmet fits your head when returned"
http://www.rta.nsw.gov.au/roadsafety/mo ... lmets.htmlwhich implies that you shouldn't use a helmet after a crash, and is a bit more official