A few weeks ago I navigated through the twists and turns of the election expenses allegations, seeking to bring a bit of clarity to a somewhat technical topic which is further obscured by a great deal of hyperbole.

One aspect of that piece was an attempt to correct some misunderstandings about exactly how the law on election expenditure really works:

‘…it’s a three-part question: were any MPs’ local spending returns incorrect; did the true levels of spending in any seat therefore breach the legal spending limit; and did any MPs knowingly submit incorrect returns?’

These are each important points – the simple question of whether a return is incorrect isn’t sufficient to to bring charges, still less to prove that any crime was committed. If an incorrect return was put in by any MP, investigators also need to look at a) the severity of the issue – did the omission conceal a candidate who spent more than the legal limit? and b) was there any knowing intent to do so? These questions matter, but are of course normally overlooked by Twitter warriors who prefer to make the most stark and garish hashtag campaign possible.

It’s important to note that we still don’t know the details of most of the investigations into the MPs caught up in the range of expenses allegations, so it’s too early to declare what the other eventual outcomes will be. A number of files are still reportedly with the Crown Prosecution Service.

But one case is revealed in today’s Guardian, which illustrates that the facts and the law do not always provide the slam dunk results that the left appear to be hoping for.

‘Tory election spending: MP admitted to police some claims were wrong’, blares the headline. The MP in question is Johnny Mercer, who took Plymouth Moor View off Labour in 2015, and the report goes on to confirm that the CPS had “found there was insufficient evidence for a realistic prospect of conviction for any offence.”

Once you delve into the details, Mercer’s case is a good example of the gap between hashtag justice and the actual law. He did indeed tell police that, on later examination, his spending return had been incorrect, but the CPS still decided not to proceed.

Why? Well, for a start the scale of the error was small. What happened was that he misinterpreted the Electoral Commission rule that “the agent must keep invoices or receipts for any payments made of £20 and above” to mean that expenditure below £20 did not have to be declared in the return, when in fact it did have to be declared, just without the need for any receipts. This meant that his declaration omitted the purchase of items like some wooden stakes for putting up advertising boards, and the printing of some paper notices about public meetings. The total amount omitted was, I’m told, somewhere between £70 and £80.

Not only was that error small, but it wasn’t enough to put him in danger of exceeding the campaign spending limit. In fact, in answering the allegations his team discovered that he had inadvertently over-accounted on other items to a greater amount, so ought in fact to have declared even less spending than he did, even had the missing amount been included.

That answers the first two questions – yes, his return was incorrect; but no, the true level of spending did not breach the legal limit. Then we have the third question: was it a knowing omission?

The fact that, even if he had included this missing amount, Mercer would have been within the legal spending limit rather answers this one. He would have had no reason to intentionally conceal spending when declaring it would have been perfectly legal. Furthermore, his explanation that he misunderstood the guidance seems entirely plausible – given (as the police report says he told them) “he had no previous political experience and no central Conservative party support during his campaign”.

All in all, this looks like an innocent error – one which should never have been made, but which has brought no attempt at punishment for the good reason that it was a negligible amount omitted without any intent to deceive or breach the limit.

As I wrote above, we don’t yet know the full facts of the investigations into other MPs, if they will fit the same profile, or how they will end. Some involve larger amounts of money, some involve more complex questions of how battle bus and other spending was allocated, and others relate to MPs whose campaigns were closer to the spending limit than Mercer’s. But his case should at least be a reminder that the volume of complaints doesn’t always match the scale of the issue – and the law, justly, requires rather more than online declarations of guilt as proof of a criminal offence.