The trial of Elisabetta and Francesca Grillo was always going to be a great media circus. It had celebrity, sleaze and, of course, Nigella. Despite not being on trial, she took centre stage as her private life was raked over. She denied much of what was suggested, but did accept taking cocaine on several occasions.

• Have you say in this week’s poll: Should Nigella have been prosecuted for drug use? – on www.thejusticegap.com

So, armed with that admission, would the police come round and feel Nigella’s collar? Well, it was announced this week that they would not be taking any action to prosecute her. Is this an example of the law in action or did she get special treatment?

It seems to me that this is what is to be expected. The police (nowadays in consultation with the CPS) have a policy that they must follow in deciding whether to take action against someone. This is the Code for Crown Prosecutors. Before any prosecution can be started there must be sufficient evidence for there to be a ‘realistic prospect of conviction’ and a prosecution must be ‘in the public interest’.

With a confession under oath, the first part of the test is not so difficult. The second is a bit more complicated. There are various factors, but they all tend to point away from prosecuting Nigella.

Firstly, this is not a particularly serious offence in any event – most people would get a fine if taken to court – it is very different to supplying cocaine. Nigella has never been in trouble before and would probably get a caution if the police had arrested her, which also points away from prosecuting.

Although cocaine use does have an impact on the wider community, there is no actual victim in this case, which is another important factor (and also means the CPS won’t have to worry about someone coming to complain about the decision that they have made).

All these features point away from a prosecution being in the public interest. There is no checklist that the CPS apply that allows you to determine whether a prosecution will or will not be started. It’s a question of weighing all the factors, and it’s a judgment call. The ultimate question is whether a prosecution (or action that could lead to a prosecution or caution) is a proportionate response to the situation.

In this case, had the police gone round to Nigella’s house and caught her in the act of taking cocaine, or she had been seen out and about snorting cocaine in a restaurant, it may have been a different story. They would have been duty bound to arrest her and the process would have started.

Why I think that a prosecution was always unlikely is that this was a ‘historic’ event, in the sense that Nigella’s cocaine use was from a few years ago. It came to light because of the Court case, otherwise it would never have been known. What is the point in going after someone for something this minor a few years down the line?

Would this apply to any ‘ordinary’ member of the public? In short, yes. The police have better things to be doing than going after someone who used drugs a few years ago. The cost of further pursuing the case (interviewing Nigella and collecting the evidence needed) would be far in excess of any benefit gained by prosecuting her.

There is one way that a celebrity may be different. It may be that account was taken of the huge publicity that Nigella faced and the negative impact on her that that caused. She has already paid a fairly heavy price in the media, and that is something that can be taken into account in deciding whether it would be necessary to prosecute her. This is an issue that most people won’t have to face obviously and I doubt it would be enough to swing the result in all but a very few cases.

I am aware of the Release report on the differential treatments that exists in policing and prosecuting those of different ethnicity. Without disputing that in any way, I don’t feel that this translates into a different treatment for celebrities (although there is not enough data to show that one way or another I would imagine).

There is another factor that is a significant one in this case. That is the fact that the evidence came to light during the trial. There is a very strong public interest in witnesses giving evidence and telling the truth. And if the message was sent out that if people answer questions honestly and incriminate themselves then they will be prosecuted, this will have a very negative impact on the justice system as a whole.

So, was the police right not to prosecute Nigella? Yes, absolutely. On the CPS own policy this was the clearly the right decision. Did she get special treatment because of her celebrity status? In my view, not. Someone else in that position would have been treated the same way.