It's the DVLA.You're screwed. Pay up and request a new V5C, at your new address and pay the £25 of whatever it is for this.If it's any consolation, I think they're c*nts too.
I got sent court papers after loosing my rag when they claimed not to have received a change of keeper when I sold the mk2.
Sorry to get you hyped up and let you down but I paid the £30 fine after my mate went through the same thing and took it to court and got a £800 fine even when he had a signed for postal slip.They get you by saying you need to chase them after X amount of weeks.