Silkman
Despite a good look beforehand, I was unable to find any information on the Boeing website - hence my post.
I've been told that it may be a 'one-off' combi a/c, fitted with a non-standard cargo door for Princess Anne's horses.
However, the information I have infers it possibly belonged to Britannia or Orion [circa 1985].
I doubt that I'll find a library now, with copies of JP old enough to shine a light on the the subject - but thanks anyway.
Any encyclopedic PPRuNers out there?