Find Ancestors

i am trying to find any relations of my gran and grandad. my gran was thomasina forest [teenee] she had a daughter,margaret,before she met my grandad george Lewis lthey got married and had a daughter,phyllis, my mother. when my mother died in 1985 we found out from a relative that my gran and grandad had actually adopted my mother,when they lived in richmond in surrey where they spent the early years of their marraige. we have came to the conclusion, we think, that margaret[my mothers older sister by 20 years] was actually her birth mother not her sister, they all lived the rest of their days in dundee, scotland.I would love to put together my family tree if anyone can help.we have tried to find my mothers birth certificate but without success and a minister told us there might not be one as sometimas was the case in those daysand the adoption might not have even been registered . We are at a standstill so if anyone could help we would be so gratefull.