Tell them to take you to court, the onus is on them to prove that you did not post it rather than you proving you did. Read thye below post, it will explain all and tell you where to stick their demand. The DVLA do not even have a legal power to issue a fine, only a demand. As a biker we have had numerous run in's with them and they have been beaten every time at court. The DVLA have a horrendous record of loosing post and this alone will win your case as long as you can say where and when you posted it.
http://www.bikechatforums.com/viewtopic.php?t=195188&postdays=0&postorder=asc&highlight=dvla&start=50Interpretations Act 1978
"7.References to service by post.
Where an Act authorises or requires any document to be served by post (whether the expression “serve” or the expression “give” or “send” or any other expression is used) then, unless the contrary intention appears, the service is deemed to be effected by properly addressing, pre-paying and posting a letter containing the document and, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post."
This basically means once you have posted it your liability is over whether or not they tell you to call them up if you don't recieve their paperwork. You don't even need to send it recorded.
