A sex party-goer who called by as a sadomasochist was allegedly strangling Pc Gordon Semple has told jurors he thought someone was "throwing up on the carpet" inside.

The man was giving evidence anonymously in the trial of former Morgan Stanley worker Stefano Brizzi, who is accused of murdering the officer and then trying to dispose of his body in an acid bath.

The witness, referred to as CD, told jurors how he was exploring fetish sex when he got in contact with 50-year-old Brizzi, who went by the profile name Domination London on gay dating app Recon.

He arranged to go to Brizzi's flat behind the Tate Modern art gallery on the evening of April 1 but got lost on his way around the Peabody Estate, the Old Bailey heard.

He told jurors: "I just remember standing in front of the building door ringing his flat buzzer.

"I was trying to understand why it was taking so long. I was lost walking around the building and finally got in touch with him after the whole experience.

"He spoke first. I remember perfectly the first phrase. He said 'hello, sorry, we are having kind of a situation here', which I found strange because I did not know what a situation means.

"He said that somebody was feeling ill or unwell and he said not to worry because they were taking care of it, whoever was there.

"I asked if there was anything I could do to help and he said no, everything was under control but our party is cancelled.

"He sounded concerned, a little bit upset. He did not sound too worried.

"To be honest, I thought when I left, somebody was throwing up on his carpet."

In the days that followed, CD told jurors they rescheduled their date for the following Friday night but Brizzi did not confirm.

Meanwhile, Brizzi was disposing of the body and may even have cooked and eaten parts of 59-year-old Pc Semple, jurors have heard.

Police were alerted after neighbours complained of the appalling stench coming from his home.

Brizzi was to tell police that Pc Semple had invited a number of people to the flat "but they didn't arrive and, when one did arrive, I was right in the middle of strangling Gordon and ... I said to him 'Look, this is not the right time now, people are falling ill and it's a mess'," the court has heard.

Witness CD said he did not realise the defendant had been arrested for murder until months later.

Brizzi, who is originally from Italy, denies murder but has admitted disposing of the body.

Later, another witness said Brizzi turned up at a meeting of Crystal Meth Anonymous (CMA) sporting a Breaking Bad T-shirt.

Brizzi was allegedly a fan of the hit TV show in which the main character, chemistry teacher Walter White, produced crystal meth and, after poisoning a rival, dissolved the body in acid.

Alexander Roberts said the defendant had worn the T-shirt to a CMA meeting and posed in it in a Facebook photograph.

Mr Roberts also said the defendant was "dark" and had talked about Satan during discussions.

Pc Semple's partner, Gary Meeks, reported the victim missing the day after he had met his death in Brizzi's flat, the court heard.

Mr Meeks made a statement to police on April 5, before the dismembered body of his partner of 25 years was found.

In it, Mr Meeks described the officer as a "very sociable person" with lots of friends at work and in the security trade.

He said he last saw his partner on the evening of March 31, when Pc Semple went to bed and he stayed up for an hour watching television. When Pc Semple got up for work as usual, Mr Meeks was asleep.

At about 2pm, Mr Meeks called to say there was shepherd's pie in the fridge for dinner and they arranged to meet later at their local pub.

His last contact with Pc Semple was to ask him to record Gogglebox on the television at about 7.45pm, the court heard.

He said: "The phone was answered but it was a muffled sound, like a pocket dial. I thought I heard breathing, I thought I heard a door. I thought I heard someone speak.

"I was concerned about him and texted him saying 'Not home yet, I can hear noises'. I was very worried. On Saturday morning I texted to say I was reporting him missing to Charing Cross."

Neighbour Steve Harris said he thought the smell coming from Brizzi's flat was like a "dead body" from the start.

Describing a doorstep exchange on April 4, the witness said: "I said to him 'excuse me mate, there's a complaint about a bad smell in the block'.

"He said to me 'I'm sorry about that, I'm just cooking for a friend, that's the reason I've got the candle on the stairs'."

On April 7, he told his brother Martin, who called police and produced an official-looking Coastal Forces Veterans Association badge to confront the defendant.

When Brizzi opened the door, he was wearing Aviator sunglasses and looked like actor Tom Cruise in the film Top Gun, the witness said.

Martin Harris told jurors the smell was by now "putrid" and Brizzi was dressed only in Speedo-style pants.

He said: "This gentleman opened the door to me. He was trying to put his glasses on and trying to put a bathrobe on. He was cool as a cucumber.

"I knew something was wrong. If you've got a smell like that, you know what it is. I smelt it before many years ago."