During the prosecution's opening statements today, the court heard how William Jones walked to Claude Road that night in April from his father's house nearby, looking for his girlfriend, who he believed to be with another man at a property in the area.
The prosecution said Mr Jones admits being in Claude Road, and knocking on the door of Peter Lewis' flat. They said he will claim that Mr Lewis threatened him with a knife, which he grabbed and hit him with, accidentally stabbing him.
William Jones denies murder, and the case continues.
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There will be a number of road closures in place in Cardiff today as the British Speedway Grand Prix returns to the Welsh Capital.
Today will be a day of often heavy showers, with the risk of hail and thunder.