Officers found 40 people hiding behind a fence in a neighbouring garden when they broke up the party, Westminster magistrates court heard.
St James, then 19, was identified as the host by her younger sister who was upstairs in bed.
“It became apparent to me that Roneth had taken advantage of her parents being away and this was the second party she had hosted”, PC Toby Harcombe told the court.
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Officers were called out in the early hours of January 1 this year, finding a house full of partygoers.
St James, who was wearing a pink dress, was identified as the host as officers broke up the event.
PC Ellie Goy said St James was “given many chances to be honest with officers and chose to lie in regards to her identity”.
She was found guilty by a magistrate of breaking the Covid-19 Health Protection Regulations, having not entered a plea, and was issued a £10,000 in a Single Justice Procedure session behind closed doors on May 19.
Kajarn Mighty-Weeks, 19, was also handed a £10,000 fine by the court for hosting a party at his Norwood home on December 20 last year.
PC Daniel Ray said he was initially blocked from entering the house, eventually finding up to 40 people at the illegal gathering when London was under Tier 4 lockdown restrictions.
“Those inside tried to close the front door on me and used so much force that the front door nearly buckled from its hinges”, he said.
He said Mighty-Weeks “tried to claim that lots lived here”, but the officer concluded: “The sheer number of people within the premises made the claims that they were all part of the same support bubble a total fabrication of the truth”.
A third £10,000 fine was handed to Cristi Tanase, 24, for a party at his Ilford home last November.
Police said he tried to claim that just 15 people were inside the property, and exclaimed “it is just a conspiracy” when told he would face a hefty fine.
Neither Mighty-Weeks nor Tanase entered pleas to the charge they faced, and were both convicted in their absence.
The prosecutions have been kept hidden from the public, but details of the offences emerged following requests for court documents by the Evening Standard.
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