The March of the Beekeepers through Parliament Square comes ahead of a major vote in Brussels on Monday. Environment Secretary Owen Paterson is expected to vote against a ban on some neonicotinoid pesticides, which campaigners say are contributing to a decline in bee populations.
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A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.