I have just received as FRA where on of the questions raised by the Risk Assessor is Confirm that the cladding meets B-s3, d2 rating.  The cladding is ship lapped pine affixed internally to the ceiling of a single story building some 25 years old and the OEMs have been lost.

The question is how without taking a sample off the ceiling do I prove it conforms to the rating?

That's the problem facing many buildings, often you can't. I'd consult someone competent in & insured for FRAEW for advice.