Around 300 people enjoyed music, dancing, crafts and sunshine during the annual May day event.
The new bandstand will be the latest stage in the restoration of the Victorian garden laid out by social housing and urban garden pioneer Octavia Hill.
"We understand that Octavia believed, as we still do, that having a bandstand was not just a place for bands to be, but a visible demonstration of a commitment towards music and performance," said BOST director Helen Firminger.
Last year BOST hosted a number of events to mark the centenary of Hill's death.
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