Thanks.
Leviathon also seems to be a fairly common spelling of the word, so that must be it.
Quentin Wilson once did a TV programme called Leviathon featuring a Jaguar E-Type, but there was no connection with this racing car.
What I have learned whilst looking is that the Jaguar Mk 10 is generally referred to in Jaguar circles as "The Leviathan", for obvious reasons!