Thanks rosie99! She was keen on this marrying lark wasn't she!!
Thanks to you, I've gone some way to solving the mystery. Mary Lizzie was 20 when she married 18 year old Bernard Parratt and their first son (also Bernard) was born not long after, followed by a daughter Alice the next year(1914)
I found Bernard's discharge papers from the North Staffs Regiment dated July 1919. Son George Parratt(aka Yorke) was born in January 1920. Unless Bernard came home on leave before he was discharged, then he can't have been the father of George, which might account for the two surnames being registered.
I then found Bernard on the Canadian Passenger list - he arrived at Port St John 16 days after his 'son' George Parratt/Yorke was born in Stoke. Bernard's widowed mother and two of his sisters had emigrated to Canada in 1913.
Back in Stoke, Mary Lizzie had a second child who was registered with two surnames - Richard, born in 1921 died as an infant. A fourth child, Irene was born in 1924 - she bore the Parratt surname.
Mary Lizzie went on to marry George Bridgwood in 1928 and then (as you've found today) William Shenton in 1959 and died aged 91.
So, it looks like Mary Lizzie had a fling whilst husband Bernard was away at the end of WWI, he came back, found her to be pregnant and so left to join his mother & sisters in Canada.
I still don't know who the mysterious Mr Yorke was, and I know that nowadays you can have a father's name on a birth certificate without being married, but I'm not sure when the rule about having to be married before a name could be added was changed?
I also haven't figured why the daughter Irene Parratt, born in 1924 is simply registered with the surname Parratt.
Still some mysteries to solve!!
Thanks again for your help