Everyone is equal before the law. There are few greater cornerstones of liberal democracy than the idea that anyone, from pauper to prime minister, can have their collar felt. When that crumbles, corruption swiftly follows. So while the excruciating details of Sue Gray’s report on Downing Street’s lockdown party culture – the flippant WhatsApp from the prime minister’s principal private secretary, Martin Reynolds, about the drinks they “got away with”, or the cleaners left mopping up drunken vomit – should weigh heaviest on political consciences, the spotlight now turns to the police. Too many of us just can’t square those pictures of Boris Johnson raising a toast at his press secretary Lee Cain’s leaving do with the fact that some around the booze-laden table got fines, and he magically didn’t. Even inside the Metropolitan police there is bafflement, with one source telling the Guardian they thought the picture “blindingly obviously evidence of a breach”. As the former director of public prosecutions, Ken Macdonald, has argued, it’s “very difficult for us to disentangle exactly how the police investigation has proceeded and how fair it’s been” unless detectives explain their thinking. Now the former Met police chief Brian Paddick and the campaigning Good Law Project are bringing a somewhat ambitious-sounding test case, aiming to make the Met spell out its reasoning over Cain’s leaving party and two other events. But in an audibly tetchy exchange with London assembly members on Thursday, the acting Met commissioner, Stephen House, would say only that his force had policed “without fear or favour”; pictures can be deceptive, he insisted, and “unless you are in full possession of the facts I don’t feel that a proper decision can be made”. What full facts could those be? Boris Johnson’s rather grand defence is that popping in to say goodbye to departing staff was one of the “essential duties of leadership”, for all the world as if he were Volodymyr Zelenskiy holding firm under bombing, not a man caught drinking during a pandemic in which children bade farewell to their dying parents via an iPad. The carefully phrased implication is that the party turned into something more raucously social (and thus illegal) once Johnson left – one way, as the legal blogger David Allen Green suggests, of building the “reasonable excuse” required in law to exempt him from Covid restrictions. But given that the table in that photograph is laden with bottles, could a vigilant boss not have guessed where things were heading and broken it up, or at least issued a stern warning as he left? Is ensuring your own staff obey your own law not an “essential duty of leadership” too? Whatever the legal definitions, it’s hard to imagine Margaret Thatcher wandering off and leaving them to it. The most troubling questions, however, concern the so-called Abba party inside Boris and Carrie Johnson’s private flat on the night Dominic Cummings was forced out, which Johnson says was actually a “work meeting” with aides about replacing Cummings – although curiously it started well before he got there, and Carrie Johnson doesn’t officially have a working role in Downing Street. Gray started looking into that, but stopped when the Met launched its broader investigation; subsequently she decided it was not “proportionate” to probe further. This is the most intimate and awkward of all the allegations, touching as it does on his home life, and remains the most sealed book. Johnson has suggested the flat was also a form of workplace, because work meetings have historically sometimes happened there, which is true. But millions of homes also became workplaces overnight during lockdown, and we didn’t all have our colleagues round for booze, snacks and The Winner Takes It All. There may, of course, be perfectly good technical or legal reasons behind all these fine judgment calls. But when I interviewed ordinary people fined for breaking Covid laws for the Guardian, again and again I heard of penalties being higher for party organisers – the student in whose house it happened, say – than for guests, in recognition of where greater responsibility lay. At Westminster, by contrast, there are indignant mutterings about junior staff who nervously confessed all to Sue Gray paying a heavier price than those with the nous to get a lawyer. And the longer the police refuse to explain their reasoning, the more people will wonder if the Met – which House said levied fines only where the evidence seemed strong enough to hold up in court if challenged – applied a higher threshold of proof in these politically incendiary cases than it did for mere mortals. The force has always given the impression of being dragged kicking and screaming into Partygate, and on one level that’s understandable. The idea of the police effectively determining a democratically elected prime minister’s fate is uncomfortable for everyone. Perhaps they’re also reluctant to get too deep into who said what on which questionnaire, for fear of setting a broader precedent: ordinary people must admit all sorts of embarrassing private things during a police investigation, and can expect some confidentiality over anything not disclosed in court. But a prime minister who breaks the law is such a unique aberration that only maximum transparency will do if Britons are to retain whatever confidence in policing they may still have. You can scrub wine stains off the walls of Downing Street. But a stain on the reputation of policing takes rather longer to rinse out. Gaby Hinsliff is a Guardian columnist