significant flutter on the exhaust on the inboard edge, covering an area of about 15 Cm, and fluttering over probably 1 Cm either side of normal position.
Nah, Snigs is right... some parts are supposed to move. The exhaust on the F27 used to move around a lot, in fact I once had one break when it was prevented from moving by a badly secured cowling. It grew a 15cm crack in 20 mins flight time... spent a day or two in GCI waiting for a new one.
More to the point, unless you actually KNOW that there was a spacer or other part missing, you should really say nothing - because inferring that a part was missing is needlessly damaging to the reputation of flybe's engineers.
I agree with
Maude Charlee - unless you immediately informed the crew of what you saw, you shouldn't be mentioning it now.