Was the trial flight 'heavy' carrying a payload?
I once worked for an operator operating, supposedly, Stage 2 DC8's and one of our 'hotspots' was Gatwick.
With an MTOW DC8-62, even with hush kits, it was never going to stay within noise limits and all they did was throttle back over the published noise monitoring points but they occasionally got it wrong.
A LPL based turbo-prop operator went even better, for a fleet of some 14 noisy turbo-props they had circa 3 Stage 2 hush kits, they would get the aircraft, by registration, certified as Stage 2 and at the next big maintenance check the hush kit would come off that one and subsequently go on another one. They probably finished with much of the fleet cerified as Stage 2 on a mere 3 hushkits
I wouldn't regard one 'staged' flight a complete success if they are planning some 30 (loaded) per each night.