I think you're doing the right thing Suzi and would just say to keep it up.
My parents divorced when I was 12 and my relationship with my father was damaged by the fact that I didn't see him for a long time (nothing to do with anything my mother did). When he did come back in to my life my mother left it for us to establish a relationship with him without ever really rubbishing him or anything like that and this has worked out very well.
I know what my father was like in the past and I know what he's like now, people do change and you're doing exactly the right thing letting the boys figure it out for themselves, (in my opinion). Keep the ground rules about the information regarding your home life but other than that, carry on the good work. Good luck to you xx