“Blood-swapping,” he says, describing it as a practice involving the injection of a drug, letting some blood out and injecting that into the other person. “It used to be taboo. I would never have had clients five years ago telling me they did it. It is a fetish for some people, a way of connecting.”

It should be stressed that most gay and bisexual men are not involved in chemsex. This is a minority within a minority, with the more extreme activities concerning yet another even smaller minority within that. There is also a scarcity of reliable, large-sample statistics on chemsex drugs usage, although the 2014 England Gay Men’s Sex Survey offers a useful indication.

Based on a sample of 15,360, 8.4% had taken crystal meth at some point, 12.5% had taken GHB, and 16.5% had taken mephedrone. When asked about drug usage in the last four weeks, these figures drop to 2%, 3.2%, and 5.3% respectively. Beyond this, anecdotal evidence from users and drug workers fills in some of the gaps.

The age range, says Doyle, is unprecedented, with people using chemsex drugs ranging from 16-year-old boys up to “people in their seventies and eighties. For the first time we’ve got drugs hitting our whole adult age group.” He suspects from conversations with fellow drug workers that underage boys are doing it, too.

We also know from drug workers and charities that it is a national issue but with epicentres in London, Brighton, and Manchester. We know too that accessing meth, G, and mephedrone is quick and easy through dating apps – in one day during the research for this story, I switched on Grindr and without me contacting anyone, two dealers sent me messages offering their products. Grindr did not respond to my request for a comment on this.

The challenge for the police and the justice system now is to encourage people to come forward. But many I interviewed were afraid, beyond not being believed or of police homophobia, that if they reported being assaulted while under the influence, they would be investigated for drugs offences.

The Metropolitan police told me, “Our priority would be to investigate the sexual offense allegation” and that it is not a criminal offence to have drugs in your system.

But a spokesperson for the Crown Prosecution Service directed me to the Code for Crown Prosecutors, in which there is one key line regarding drug users as witnesses to a crime: “The more serious the offence, the more likely the witness…will be prosecuted.”

Given that supplying drugs is the greater offence than possession, that should reassure victims. However, simply passing a needle or a glass containing G – routine in chemsex – to another partygoer could constitute supply.

The silence surrounding what is happening, agreed the interviewees, is compounded by a paucity of services available. There is some specialist help for those suffering from problems caused by chemsex, at sexual health clinics such as 56 Dean Street in central London and with some LGBT charities such as Galop, London Friend, and the addiction organisation Antidote. But these can never cater to the sorts of numbers that might need it.

The day Paul Doyle comes to see me, he had received some news about his residential unit in Cornwall, the only one of its kind: “I had a phone call this morning to tell me it won’t be staying open much longer.” By January it will be gone.

Meanwhile, says Doyle, drug counsellors across Britain continue to phone him, needing information about chemsex because they know so little.

There is one final detail that he describes, about where on the body chemsex users are often injecting the drugs, which reveals more than it should. “Stabbing it in their groin, their feet,” he says. “Places that are dangerous. Places that can’t be seen.”

But it was what was in the needle that led Rob into the canal that Saturday afternoon. Two days before, he had invited a man who he met online over to his house. “His only interest was to get the chems inside me. There was no foreplay, just, ‘I want your arm, let’s get this into you.’” Rob agreed to take half a milligram of crystal meth.

But the man, whom Rob describes as much bigger than him, kept pushing the syringe. “When the 3.5mg went in my whole brain started…he could tell that really hit me. I was scared. His clothes came off as quickly as he possibly could and said, ‘Time to get up inside you, mate, you’re never going to forget this.’”

The man started putting his hands round Rob's throat while penetrating him, he says. “I fought this as if to say, ‘That’s too much.’ I hadn’t gone under enough for him to do anything without me belting out, so he said, ‘Ugh, I can’t do anything with you.’” The man stopped and got up. Rob was shaking but eventually the man went to leave. His parting words, says Rob, were, “I’ll turn up somewhere where you least expect me to.”

It was only after the man left that Rob looked around his bedroom – where we sit now – and saw what cemented his belief that the man had wanted him dead. “There were two full syringes of crystal left.”