I've managed to find some more information out that will help answer/eliminate a few possiblities. I visited my mum and we had a look through her stash of family paperwork and letters. We established that Dorothy had 2 main guardians in her childhood - Mrs Soper (and her daughter Cissy) and Hilda Legg.
From information within in the paperwork, I have managed to find their full names and found them in various records including on the 1921 - but Dorothy was not with any of them!