don't worry I worked it out, excuse my laziness, I just used:
<li{if (($smarty.foreach.product_tabs.last)&&($product.pr oductid eq "17516" or $product.productid eq "17515"))} class="last_tab"{/if}><a href="#{$prefix}csi-{$tab.tabid}">{$tab.title|wm_remove|escape}</a></li>
again many thanks!!