Once again many thanks to everyone for their suggestions, which have helped me clarify Susan's life, though clearly there are some matters that remain a mystery.
I think that I have Susan traced in every census except 1881, recorded as Susan Stone until 1891 and then as Susan Pennell (widow) in 1901 and 1911 passing away in 1911.
I'm assuming that son William (1862) is not the son of the mysterious Mr Pennell as he is recorded as Stone in 1881 whilst his half brother is recorded as Teddy Pennell.
Teddy's birth was registered as Edwin Stone (dob 20 December 1875) and the military record of Edwin Pennell dated in March 1894 aged 18 years 4 months is consistent with this and also shows his mother as Susan Pennell and father unknown.
Whilst I do think that the record for Susan and Sarah Stone in Yeovil in 1891 is the sisters, I'm a bit concerned because sister Sarah's age is massively out with her birth year recorded as 1857 when she was, in fact, born in 1838 and was older than sister Susan.
I was amazed that the children William and Edwin would have been sent for "correction" as far away as Surrey.
I'm assuming that Susan never actually married the unknown Mr Pennell but don't quite understand why she would not identify herself as a Pennell until 1901 - perhaps because she waited until he was dead?
Maud remains a bit confusing. Whilst I recognise that children were often recorded as sons or daughters when they were grandchildren for various reasons, I don't quite see why Susan would record Maud as a daughter in 1901 when she was happy to record another child in the household (William) as a grandson.
Also if Maud was a granddaughter, I assume that Susan had another as yet, unidentified child who was Maud's parent.
Thanks again for you all of your help.