I call them by their names. They did ask me to call them Mum and Dad but I really wasn't comfortable with it so after 5 years still haven't done it and am pretty sure I won't. It's nothing against them, I just don't feel comfortable and I think they understand that now.
I suppose the best thing to do is ask them what they'd like to be called. I don't call my inlaws "Mum" and "Dad" because I already have a Mum and Dad and that would be confusing and seem a bit phony. I call them by their first names.