By a fortunate coincidence I've just been looking at the 1911 census for the 'London' Ditch families - actually my direct line is from the Ditch families of East Sussex, but since I've been researching the surname for about 25 years, I've also become interested more recently in the Chelsea Ditches.
There are a few interesting questions surrounding the families you are discussing...
1911 census lists Alfred Ditch (I believe that is a typo error in your message Helen?) his wife Mary living at 14 Fords Place, Battersea, London. I've located an Alfred in the birth indexes for Chelsea district, registered in December qtr 1864, and am fairly confident that's him.
Of the 13 children listed (!) in the census, sure enough Susan Ditch (daughter) appears, aged 22, which would also fit - but the odd thing is she's listed as being married - yet with the surname Ditch, not Prince. So based on your information of her marriage to George Thomas Prince in 1907, I decided to search for him in the census, and sure enough, he appears, with Susan and their 3 children (Lily, George & William) NEXT DOOR to Alfred at No 13!! Someone in Alfred's household must have listed Susan there and forgotten she was actually now Prince in name - most likely the families were in and out of eachothers' homes constantly!
So to Alfred's origins - firstly by cross-referencing a marriage search on the FreeBMD site I discovered his marriage to Mary REED (Dec qtr 1883) in Kingston district - this must be them since Mary is stated as born in Kingston on Thames in the 1911 census.
Alfred's parents: Thomas & Mary/Marian I believe this was Thomas' second marriage, his first wife being Sarah Agnes Silcott as Sheila has already mentioned. There is a marriage of a Mary Ann (hence 'Marian'?) HALLETT and Thomas Ditch in 1861. The 'clincher' comes in the 1871 census where Alfred appears alongside his half-brother Edward.
Andrew Ditch (son-in-law) I believe to be one of the children of a previous marriage of Mary Anne Hallett. The others appear in the 1861 census. I haven't managed to determine Mary Anne's maiden name.