A seven-year-old boy swims 21 miles to help other children.
Joseph Purcell decided to act after seeing television pictures from war-torn Syria.
So the youngster, who is from Wokingham, persuaded sponsors to back him as he tackled a distance equivalent to the English Channel.
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The plaque at number 16 Cowley Street in Westminster marks the brown brick Georgian townhouse where Sir John lived for 31 years.
A man reportedly armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.
Metropolitan Police officer, Tom Harrison, has been crawling the course since the race began on Sunday morning.