By Gemma Strong The late Queen would treat herself to a favourite tipple before bed, according to her cousin Margaret Rhodes. While the monarch was known to be partial to a gin and Dubonnet, she would reportedly unwind at the end of a long day of engagements with a glass of bubbly. Margaret, who passed away in 2016 at the age of 91, was well placed to know. As well as being first cousins with the Queen, they were very close childhood friends and she acted as a bridesmaid at her 1947 wedding to Prince Philip. It’s not known which brand of champagne Her Majesty preferred. But it was almost certa…