Ok I tried bernaise sauce again tonight, just for an experiment, and I think I may have found the controlling variable - Lurpak butter!! My wife and I worked out that the recent failures all involved Lurpak whereas with the previous attempts we just used woolworths homebrand butter.
I'm not going to close it off there, I will try another couple of variables to be sure.
I would love to hear from those that have continuous success - do you ever change the brand of butter, temperature of the butter (ie softened or straight from the fridge) or the order in which the ingredients go into the mixing bowl?
I know it's a boring subject but I want to have one of my big clients over for dinner and I need the confidence to know I will get it right!
PS After comparing the ingredients, Lurpak has lactic acid culture which homebrand does not. Maybe that is the answer?