Lucky you didn't have to make a claim, though with NFU once you've been with them for 5 years (so they can check you out) always send out your new insurance certificate with your renewal...
Question is about your complaint. Are you just complaining to make a noise or have you specifically asked them for something - a formal apology; 3 further months free cover; if they accidently forget to renew your insurance again they will be still be liable whether you pay or not or assurance (or free insurance) it will never happen again?