I never realised this: Marie Rose sauce got its name from a Royal Navy chef feeding the crew that brought the remains of the wrecked Mary Rose to the surface in the early 1980s (she sank during battle in the Solent in 1545). The chef had nothing to go with the prawns so was forced to improvise …
Hard to imagine ketchup and mayo didn't get it on before the 1980s – but in any case, I think our explanation holds more water. PW