Hi,
New to the forum but not new to the DVLA
The answer to should you pay...
Well yes as the "lost in the post" will not work even if you have proof of recorded delivery this is not always accepted unless the DVLA have written down what was in your package accurately otherwise you have no proof that you sent your SORN/ Disposal Notiication to the DVLA only that something was sent.
The other option, if you wish to take a risk is don't pay.
The DVLA are no longer taking people to court over this offence instead are using debt collection services at the moment these are only taking on "clean cases" ie case where the accused has not written in, phoned or made any contact with the DVLA regarding the fine. This is not to say in the furture they will not start picking up these cases as they have 6 years to do so.
It's a risk and is it worth it over 40 quid, if it is don't pay, you won't be taken to court it'll be a big guy calling at your door!
Hope that helps
x