Horace Walpole would not have known Queen Anne as she died in 1714 and he was born in 1717.
His father, Robert, definitely did, and would have attended numerous court occasions. Apart from that, the Walpoles moved in circles which had been involved with the court and politics for a long time, so Horace would have received first hand knowledge of Queen Anne's jewels etc.
For a brief explanation see Horace -http://en.wikipedia.org/wiki/Horace_Walpole Robert -http://en.wikipedia.org/wiki/Robert_Walpole