Hello

Is there anyway to search for my grandfathers father, even though he doesn't appear on any marriage or birth certificate?
My grandfather found out his sister was actually his mum when he was about 20 and the father is unknown. Sadly my grandfather is no longer around and he hated the fact he was illegitimate, so the question never came up.
Anyway to find this information out?
It was around Cambridge and my grandfathers last name was Watson.
Thanks.