If anyone is interested, in a google hunt I found this recipe:
http://www.gastronomydomine.com/?p=345

Interestingly enough, that recipe is close to the Fannie Farmer recipes
however they differ in FF using a fresh egg and the online recipe using a
hard boiled egg.

I also found that apparently Heinz invented Salad Cream especially for the
UK market in the 1920s so sf was quite right (although now i'm wondering why
ti was that FF has 2 recipes for it) apparently Mr Heinz product contains
far less fat than a normal salad dressing.