What are you hoping to achieve with a statutory declaration?
You seem to accept the offence?
Were you driving?
Ok. Were you convicted in absence of speeding or failing to identify?
It is easy enough to make a statutory declaration. Usually you just attend at about 9.30am at the relevant magistrates court and ask for it to be listed. It is worth calling in advance as some courts have specific days upon which stat decs can be made. There is sometimes a small fee.
The fact that the correspondence was not received for good reason is a ground for a statutory declaration although they might just reopen the prosecution anyway.
That still leaves you with the outstanding charges though. Almost certainly they have charged you with failing to identify the driver. It might be a stand alone count. It might be that you are jointly charged with speeding. If so, then they will probably accept a plea to speeding alone.
Can I clarify anything for you?
Because the duty is upon the registered keeper to make sure the address is up to date with the DVLA. That is usually the basis for the argument.