Thanks for the info Jac. There's a lot of bidding on it already for an auction that has another 7 days to run so someone knows what it is or they want the heads only.
The heads alone I reckon are worth the current level of bidding plus a bit more, but as a combo, it is an odball to be sure. The seller was not forthcoming as to the previous application so I am kind of at a loss to understand what this was used in up to 2002 when it was allegedly rebuilt for £5K.
My best guess is that it was used in some kind of circuit application, possibly Mustang/Falcon? But it's a wild punt in the dark really.