Finally got around to publishing the Consangunity page of my database that lists relationships in order of consanguinity ie how closely couples were related.
One Uncle Niece relationship (25%): Ernest I Duke Saxe-Coburg-Gotha 1784-1844 and Marie Württemberg 1799-1860. Surpringly late in date.
There are two half-uncle niece relationships further down the list:
John 3rd Baron Darcy Aston 1602- and Isabel Wray -1623
Christian August Oldenburg I Duke Schleswig-Holstein-Sonderburg-Augustenburg 1696-1754 and Frederikke Louise Unknown.
George, Duke of Clarence, and his younger brother King Richard III also listed since the Neville sisters, Isabel and Anne, daughters of Warwick the Kingmaker were their 1st cousins once removed, 2nd cousins once removed and third cousins.