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From Engineering Timelines : "The present St Paul's Cathedral, designed by Sir Christopher Wren, is the fourth cathedral on this site. The first two Anglo-Saxon buildings were timber, and the third a Medieval stone structure with a timber roof. All three were destroyed in turn by fire — the third one was destroyed in the Great Fire of London."

Spitalfields Life has a good post: Relics Of Old St Paul’s At New St Paul’s.