Linda many thanks. Your reasoning is pretty convincing and I was coming to the same conclusion on balance. I suspect I need to look earlier for Harriet Thomas' death - maybe in the mid 1830s. The logic means that Sarah Cooper had to pretend in church she was Harriet Thomas but if they fudged marital status on a birth certificate then fine. The other thing is that the older children and Adelaide in particular must have been in on it too.
In the 1851 census which is bewildering, I took the occupation of L Thomas to have been Brokers Man - a bailiff in other words a la Cinderella the pantomime characters. However the Berkshire fits and I am going to have a trawl around Berkshire to see if Sarah Cooper might have had male family.
But again thanks.