Honestly, I doubt anything will happen. If you've graduated and recieved your degree certificate, then you're in the clear. It will probably cost more than £14 for the university to track you down and send you an invoice for the money and it certainly isn't worth taking you to small claims for. Universities make hundreds of thousands of pound every year from library fines, so they probably would only do something if noone paid them. I'm also sure it's frowned down upon now for Unis to withhold degree certificates for non-academic fines, so it really is in their best interests to forget about it.
You might not be allowed a graduate library account though!