His speed record of 400.22kph/249.09mph was set on 11 August 1986 and was for an ordinary helicopter (nothing other than the main rotor to pull it along) and still stands. It was achieved during the BERP (British Experimental Rotorcraft Programme) of those days.
Among other things the BERP programme looked at blades with a wide chord at the tip. The wide chord (and thus lower aspect ratio) gave a larger stalling angle (to counter retreating blade stall) and allowed an increased mach number (before shock formation) for the advancing blade thanks to the reduction in the thickness chord ratio at the tip.