Kew Gardens has been closed to the public "as a precaution" after a severe weather warning for London. It comes after a woman was killed by a falling branch on Sunday afternoon.
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The mayor claimed last week the bridge would cost more to cancel than to build. He said £37.7m of public money had already been spent.
The houseboat in Chelsea, west London, comes complete with three bedrooms, three bathrooms, a reception room and its own patio.
The city is one of a small number which has seen an increase in traces of the Class A substance over recent years.