If it is longer than 21-days ago then the court doesn’t have to accept the declaration, unless there are compelling reasons to do so.
Perhaps one of our members more knowledgeable in this area can advise further.
A late statutory declaration may be accepted if "it appears to the court that it was not reasonable to expect the accused to serve that statutory declaration within that period".
This is, from what I know, relatively generously interpreted.
From previous threads here it seems as if courts are sometimes responding to people contacting them saying they didn't know about proceedings against them by scheduling an appointment to make a statutory declaration at a time that's convenient to the court and its calendar, which could be some time after the initial contact. It wouldn't be appropriate to choose not to accept the declaration at the time chosen by the court, rather than by the defendant, if the defendant was making diligent attempts to figure out what was going on immediately after receiving the next steps notice.
I think you might be conflating two separate issues. You are correct to say that the overburdened courts system is sometimes inviting defendants to make SDs in court weeks or months hence. When a defendant then makes the SD out of time, it is clearly within the court's power to accept it as the court's schedule meant to defendant could not reasonably have made it any earlier. However, where a delay past the 21 days is caused by the defendant, whether that be by sticking the head in the sand after getting a notice of fine and collection order and then only starting to act when they get the further steps notice or otherwise, that may result in the court finding it has no jurisdiction to receive the statutory declaration.
It would make more sense for courts to tell defendants that they can make the declaration in front of a solicitor etc instead, but they don't seem to be doing that.
The fact that the solicitor option incurs a fee is a potential reason not to do this. Potential for Daily Mail scandal of "innocent defendants have to PAY to lodge appeals because courts say they're TOO BUSY" etc.
And some courts are scheduling hearings to re-open a case in the interests of justice, which strikes me as even more wrong, but they get to decide what their administrative response is!
If the court decides that the defendant's delay beyond 21 days in lodging their SD exceeds what is reasonable, the court has no power to accept the SD. If nevertheless the court considers that the interests of justice require that a case be reopened, the section 142 procedure is what it needs to use.
One final point not a response to anyone in particular – when lodging an SD against a conviction where the case was commenced by Single Justice Procedure Notice, the defendant must at the same time serve a response to the SJPN (section 16E (3) (d) MCA 1980). The submission is not complete without one. A response can include a plea of guilty with or without a wish to attend court, or a plea of not guilty/a wish for the case to be handled outside the SJP. Not including this response means the conditions of section 16E (3) have not been met and the proceedings where the defendant was convicted in absence will not be voided. This response is mandatory whether the SD is completed before a solicitor, a magistrate, or otherwise.