She's not interested in him at all. I think she'd be upset with me if I ever told her I was interested in knowing more about him. I only found out about him when I as 20 and needed a passport, so she gave me her adoption certificate which labelled my Granddad as her adopted father (my Nan is her biological mother though). Mum likes springing information on people randomly 
All I know about him is that he left after Mum's sister was born, Mum was a toddler. I would just like to know who he is really, not interested in a big relationship with him. I would also like health information as my other three biological grandparents have all had problems (Dad's parents both died before I was born and Dad doesn't really talk about them, or want too).
I do realise it'll probably be disappointing, last year my long lost aunt and her two children got in touch and it wasn't what I'd expected at all so I have no hopes for this guy whatsoever!
WIBU to try find him even though it'd probably piss off my Mum and Nan? Would I even be able to find him? I have a surname and the names of his two daughters with my Nan, and my Mum and Nan's birthday's.
I wouldn't dream of telling this guy anything about where my family live, or my full name or use my normal email address, etc. Mum's privacy would be dealt with carefully.
May not reply until tomorrow! Going to bed soon! TIA