The couple, who have been together for three years, met in Guardsman Addison's home town of Grimsby in Lincolnshire, where his parents Mark and Alison live.
His excited mother said: 'To see him propose at the Palace was tremendous. Marta is a lovely girl.'
Steve King, 33, from Reading, proposed to Jenny Parkinson, now 29, during a party also attended by the Duchess of Cambridge.
Like Miss Slowinska, Parkinson promptly said yes.
Others to make the Palace the backdrop for a romantic proposal include Dean Nicholls, 27, who proposed to his girlfriend of three years, Lauren Green, 28, outside the front gate of Buckingham Palace.
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