Confession No. 1: The sauce made perfectly, until I put in the lemon juice and then the d**n thing split! So on the basis that it is just hot mayonnaise, I got another egg yolk and a clean bowl and gradually whisked the split sauce into the egg yolk over the simmering water. It worked a treat!
Confession No. 2: I served this to my husband, he tasted it and said what a nice CHEESE sauce it was. I don't know why I bother sometimes!