I'll just answer this for clarity (not for an argument). This applies to SDs made in a Magistrates' Court in order to set aside a conviction where the defendant was not aware of the proceedings.
When a person comes before a court to make such a Statutory Declaration the court must hear it if it is made within 21 days of the defendant learning of the conviction. The date they learned of the conviction is taken from their own declaration. It actually forms part of it. If you read the form which I posted in post #32 on Monday you will see what it says. I have attached an extract for you. The declaration (including the date the defendant found out about the conviction) is made verbally to the court and one of the Magistrates signs the form to certify it has been properly made. The declaration in its entirety is not challenged and no enquiries into its truth are made by the court.
If a person wants to make an SD outside 21 days the court may agree to hear it. The decision is theirs (that is the only decision they are called upon to make when it comes to an SD). They will agree if they believe that the defendant had good reason for making a late declaration. One of the most common reasons for lateness is where the defendant was unable to be given an appointment with the court within 21 days. In those cases the late declaration would be accepted almost invariably. In other cases they will listen to the defendant's explanation. They may question him on the circumstances surrounding the lateness in order to make their decision. If they agree to hear it then it is made in the same way as one which is made in time - that is, unchallenged.
In either case, once the declaration is made its effect is to set aside the conviction and any penalties that were handed down. There is no further enquiry; the SD, having been properly made and certified, is all that is needed. So the answer to your question is "yes". Once such an SD is made and certified it automatically and immediately renders the previous conviction void. In practice it may take a few days for the information to reach the necessary offices (e.g. the DVLA and the court's fines office) but the conviction is void as soon as the SD has been certified.
The situation is slightly different where an SD is made elsewhere (say before a solicitor) but that need not concern us here.
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!