Thanks for that mshrmh,
And I must now stop casting aspersions towards those two elder sisters, Annie and Alice Violet, as I've just now received Patricia CLARKSON's birth certificate of 6th March 1927, her parents' names clearly given as John CLARKSON and Alice CLARKSON, formerly CASEY. So the long, 14-year gap between siblings was simply a strange fact of nature/anomaly, possibly caused by the intervention of The Great War and its long-lasting after-effects, perhaps...
regards, keith