This is a Significant Record milestone.
The modifed Westland Lynx AH.1 (registration G-LYNX) achieved the absolute speed record for a rotorcraft (216 kts/400.87 km/hr), piloted by Westland Chief Test Pilot Trevor Egginton with Derek Clews, Flight Test Engineer. The record course covered 15km from East Huntspill to Hartlake Bridge in Somerset, England - see FAI Record Category E-Absolute (Absolute Record for class E - Record ID 11659). A summary of the background to the record attempt is provided in an article in the Sep-Oct 2016 issue of Vertiflite, and "Putting the Record Straight" by D. Gibbings.
The aircraft was extensively modified, including BERP III type blades and 1,200 shp Rolls-Royce Gem 60 engines, amongst many other modifications. More details of the aerodyamic technology behind the record can be found in the following reference,
Ref: The Aerodynamics of the Helicopter World Speed Record, F. J. Perry, Westland Helicopters Ltd., Presented at the 43rd Annual National Forum of the American Helicopter Society, May 1987
The aircraft is now on display at the The Helicopter Museum in the United Kingdom.
See also the following resource for more information about helicopter speed records,
"Helicopter Speed Records", AHS Staff, AHS web page, last accessed 14 Aug 2017