I had same thing happen to me I sent logbook off and they didn't receive it, they wanted proof I posted it and I said to them I did what I'm supposed to do fill in logbook and send it to dvla if you did not receive it that's not my fault get onto royal mail. They then again asked for proof to which I said can you supply me with proof to say that I didn't post it! I had letter couple days later dropping charges. They state fill logbook in and return it, which many people do they can't fine people for royal mail losing it, or even Dvla themselves. We shouldn't have to send items recorded but now tbh I do as its not worth the hassle hope this helps