To Graeme's Questions:-
Civilian? British? Turboprop twin? Though I don't know why the scoop would be so low down the nacelle?
You replied:-
I'll add that at least one of Noyade's questions receives the answer 'no'...
If that were British? No. Coupled with the statement :-
this is a very rare example of the type in that it has a leather 'luxury' interior
Just a very wild guess the Hawker Beechcraft King Air B200GT