The chief constable of the police force examining claims against Angela Rayner has defended the decision to investigate the Labour party deputy leader, vowing it would be done “fairly and impartially” and would establish whether “culpability is proven or otherwise”. Stephen Watson, who leads Greater Manchester police (GMP), gave his most extensive public comments so far on the investigation, which was triggered by a complaint from a senior Conservative. Watson’s interview with the Guardian was given to mark his third anniversary leading the force, becoming chief constable when it was failing so badly it was in special measures, and is credited with dragging it out of crisis. The decision by his force to investigate Rayner – who denies any wrongdoing – brings policing and politics potentially into conflict again. GMP first said it would not investigate claims against Rayner over whether the house she once lived in Stockport from 2010 was her main residence and whether she thus may have avoided tax when she sold it. The Labour leader, Sir Keir Starmer, has called claims his deputy lied about her primary residence to avoid tax a “smear”. Watson told the Guardian it was a letter from the Conservative deputy chair, James Daly, that led to his force’s dramatic reversal. He said: “We received an initial piece of information, which was assessed and led us to a certain conclusion. “We then subsequently received a letter, it’s a matter of record, from James Daly. The information within our letter caused us to reassess whether in fact an investigation was necessary. That is now ongoing …” He added: “Where crimes are reported to us or potential crimes are reported to us, we have a process of assessing what we know, what we think we know and what is being asserted by others.” A key stage in this process, once police have finished their investigation, would be to submit materials or files to the Crown Prosecution Service (CPS) to consider whether criminal charges should be brought. Watson made it clear he did not want to discuss the investigation, but seemed to indicate that it was by no means certain it would reach the stage of being sent to the CPS. Asked if the investigation would end up with a file going to the CPS , Watson said: “Not necessarily. Depends what comes out of the investigation … We’ll see where it ends up.” Daly, whose complaint led to the investigation, has in media interviews declined to say what offences he alleges Rayner committed. Watson declined to confirm claims Rayner would be interviewed under criminal caution. Asked if there was a trend of politicians trying to involve the police to undermine their rivals, and whether that bothered him, Watson replied: “Not particularly … people may assert that … my job is a simple one … We investigate allegations of crime. “We do so fairly, impartially. And we go where the evidence leads us. “Of course, we operate in a system and one always has to be careful that we’re not drawn into political spats, all the rest of it, which is why candidly, I’m very loath to make comment … because all it does is it plays into that space and that for me, is not the role of the police. We should just simply play that stuff with a straight bat …” He added: “Ultimately, of course people have a responsibility as to how they conduct themselves and we do too.” Watson said: “People make allegations, we have a look at the facts and we come to a determination as to whether culpability is proven or otherwise. “And we just do that in a time-honoured fashion. We don’t always have to reach for great innovation in this space. We just do what it says on the tin.” Watson cited a litany of statistics to show his force had dramatically improved, such as answering emergency calls as quickly as other forces, catching more suspects than before and failing the public less. He hailed his “back to basics” approach. Watson was reputed to be the favourite chief constable of Suella Braverman when she was home secretary. He said his focus on the basics was in contrast to some police chiefs who forgot their main mission was to fight crime. Amid continuing debate about police performance, Watson said GMP’s former leadership had previously made “stupid decisions”, with a giving up culture entering the “DNA” of the force. He said the improvements made in the force had largely been made with the existing workforce and said until the basics had been fixed, police chiefs were “not morally entitled to whinge to the public that we can’t cope with demand”. Watson said: “We live in times where police forces can quite quickly forget the essence of what the public pay us for and we can go down all manner of rabbit holes and I do feel that this is a real danger.” He said the damage from headline-grabbing scandals involving the Metropolitan police officers Wayne Couzens – who murdered Sarah Everard- and the serial rapist David Carrick, were made worse by a public feeling that police are poor at solving crimes. “My problem with not doing the basics well is that these things start to be seen not as aberrant one-offs, but as corroborative of a view that has already been forming in people’s minds that policing is just not really very good at doing its core. “I take the view that ultimately the only way to win back confidence is to do the basics brilliantly all the time,” said Watson.